RSS readers - Windows

Omea - one of the most fully featured RSS readers out there; can get a little slow as its database grows; free in its basic version, also has a commercial, advanced version with more features
Mozilla Thunderbird - Mozilla’s e-mail and RSS reader, supports skins and add-ons
Awasu - RSS reader with a customizable user interface, the ability to synchronize with other readers, bookmarking and annotation, and republishing of your feeds
GreatNews - integrates with Bloglines, and offers a very fast way to browse through your feeds
RSS Bandit - RSS reader that offers organization of your feeds through folders, as well as synchronization across several installations
IntraVNews - an RSS reader that integrates with Microsoft’s Outlook.
SharpReader - requires .NET framework, lets you organize feeds into folders, integrates with Feedster, can display new items via system tray popups
eCruiser - an RSS reader in Japanese
Juice - a pretty basic RSS reader
Opera - Opera web browser comes with a fully featured RSS reader
FeedDemon - Newsgator’s feed reader offering a high degree of customization, newspaper-style feed reading, synchronization with other Newsgator products, and feed search
Snarfer - a free RSS reader, with key features being river of news style view, embedded video display, synchronization with Bloglines, powerful search and filtering
Active RSS Reader - simple RSS reader with a very small hard disk and memory footprint
Feed Amasser - a very lightweight, freeware RSS reader
FeedExpress - a freeware, open source RSS reader; quite basic, but with some nice features, for example CSS customization
Crackfeed - a tiny systray based application that notifies you with RSS news as they come
HappyFish - RSS reader and podcast client, requires .NET 2.0
Attensa for Outlook - RSS reader that fully integrates with Outlook, support OPML import/export, scheduling, republishing, and advanced feed organization
Briz RSS Reader - a very simple RSS reader
Particls - desktop RSS notifier and ticker with lots of options, also has a Firefox plugin
Anothr - RSS reader/bot for Skype/Gtalk/MS

RSS readers - Linux

Liferea - RSS reader for Gnome users
Akregator - RSS reader for KDE
Straw - RSS reader for Gnome, supports RSS and ATOM, as well as exporting/importing OPML feeds
Snownews - text based RSS reader for all you 1337 people
AgileRSS - desktop aggregator that is able to display any RSS, ATOM, and XML news feed; also offers an RSS news ticker

RSS Readers - cross-platform

RSSOwl - works on Mac OS X, Windows and Linux. Offers, among other features, a multi-pane, highly customizable interface, OPML and blogroll import, internal browser, and a search engine.
BottomFeeder Rich-featured multiplatform RSS reader (works on Linux as well as Windows and OS X), supports CSS customization, OPML import/export, and plugins.
BlogBridge - open source RSS aggregator that allows you to create dynamic feed reading lists

RSS readers - web based

Google Reader – In addition to tracking your feeds, Google’s popular RSS reader allows you to star items (like in Gmail), view trends in your feeds and reading habits, and recently added an option to read feeds offline.
iGoogle – Formerly known as Google Personalized Home, iGoogle allows you to include RSS feeds as part of your custom start page.
Netvibes - This personal homepage provider includes hundreds of “modules” you can add to your page from Hotmail to your Flickr photos, but also lets you create your own modules by simply entering an RSS feed URL.
Pageflakes – Another personalized homepage provider, Pageflakes functions similar to Netvibes, letting you select from pre-made modules or create your own by adding an RSS feed.
My Yahoo! – Still the most widely used custom homepage on the Web, My Yahoo now allows you to add RSS feeds alongside your weather, stock quotes, and news. Similar to Netvibes and Pageflakes, My Yahoo allows you to re-arrange your feeds with drag and drop features.
My Netscape – Netscape’s personal homepage service also includes the ability to add RSS feeds as content modules. An account also lets you participate in their Digg-like homepage voting.
Daily Rotation – A no-frills personalized homepage aimed targeted at techies, Daily Rotation includes recommended feeds from hundreds of tech sites, and also lets you add in additional feeds of your own choosing.
Rojo – A part of Six Apart, Rojo is a web-based reader that combines RSS aggregation with a community of sorts, showing what people are reading on their homepage.
Bloglines – One of the early online RSS readers, Bloglines “split screen” interface makes it easy to view the full content of any of your feeds. The service is part of the IAC family of sites.
Newsgator – NewsGator’s RSS reader allows you to organize your feeds in Windows-style folders that can be expanded and collapsed.
MySindicaat - an advanced online RSS reader with a tree-like feed layout, available in several different flavors, including a mini ticker and a mobile RSS viewer. It also has RSS mixing capabilities.
Fwicki - web based RSS reader that lets you create custom river-of-news style feeds.
NewsAlloy - a fast and full-featured web based RSS reader. Also has a mobile version.
Feeds 2.0 - a personalized RSS aggregator, that learns from your reading habits and displays relevant feeds

RSS to email converters

Feedblitz - one of the most popular RSS to e-mail converters. Supported by FeedBurner. Also has a Firefox plugin.
ZapTXT - monitors feeds for certain terms and sends you notifications via e-mail. Also has a mobile version.
R|Mail - offers simple subscription to any RSS feed, as well as an R|mail subscription widget which you can use on your site

RSS Feed Validators

Feed Validator – By entering your feed URL at Feed Validator, the service will literally translate your RSS to code and identify any potential problems. It works with versions of RSS up to 2.0.
RSS 1.0 Validator – This validator for RSS 1.0 generates an HTML page identifying any errors.
Redland RSS 1.0 Validator & Viewer – This validator offers a variety of display options for checking your feeds.

RSS-related plugins - Firefox

Sage - RSS reader for Firefox, has a decent amount of options, allows CSS customization
Wizz - a very advanced RSS reader for Firefox with a huge set of options
RSS Ticker - an RSS ticker for Firefox with a large number of options
Beatnik - simple RSS reader for Firefox
BlogRovr - a Firefox plugin that brings you related news based on your feed choices and the website you’re currently visiting
NewsFox - fully featured RSS reader for Firefox, with three panes; similar to Thunderbird
InfoRSS - another RSS ticker for Firefox with a very comprehensive set of options
Feedbar - Feedbar displays new live bookmarks items directly in your Firefox sidebar

RSS-related plugins - other

Good News Trillian RSS plugin - allows you to read RSS feeds in Trillian
RSSNews - RSS reader for IM client Miranda
RSS Feed Scanner - Azureus plugin that allows for automated downloads by means of advanced feed filter configurability
RSS Popper - RSS plugin for Outlook and Outlook Express
RSS Reader for Gaim - enables you to read RSS feeds in Pidgin (formerly Gaim)

RSS managers

Feedburner - the obvious one. Recently acquired by Google, FeedBurner has become the de facto standard for RSS feed management, offering a huge set of features, including detailed analytics, chicklets, feed optimization and ad insertion.
WebPasties - a set of RSS-related tools, including some feed management features
FeedPass - creates a nice landing page for your feeds

RSS mixers

Yahoo! Pipes - an advanced RSS mixer with a graphical user interface; supports simple operators, filters, and many advanced functions
Feedbite - a combination of an online RSS reader and an RSS mixer, with the possibility to vote for RSS “bundles”
Feedblendr - a very simple RSS mixer that doesn’t require registration
Blogsieve - full featured RSS mixer that lets you blend up to 5 feeds and do some advanced filtering on the results
Feedcombine - RSS mixer that lets you blend feeds from a list of predefined sources
Feedshake - RSS mixing and some basic filtering
xFruits - a set of very neat tools for RSS blending, filtering and conversion, including an RSS to PDF tool
BlastFeed - an RSS mixer that lets you combine and filter feeds and get them delivered via e-mail, IM or RSS
FeedRinse - an advanced RSS filter that enables you to get rid of the “spam” in your RSS feeds
FeedDigest - RSS mixer that lets you mix and sort feeds and then republish them on a website
Popfly - Microsoft’s very promising RSS mixer, currently in closed beta
Google Mashup Editor (GME) - Google’s set of developer tools for creating mashups
RSSMesh - a PHP script that lets you blend several feeds into one
Afeeda - RSS mixer which lets you blend all your favorite feeds into one

Feed directories

Syndic8 - one of the biggest RSS feed directories; integrates with several services on this list
Technorati - indexes blogs based on tags and authority, as measured by incoming links.
Feedster - organizes feeds into content channels such as technology and celebrity gossip.
Blogstreet - places feeds into different directory categories and also has a Digg-like homepage powered by user rankings.
Weblogs.com - one of the original feed directories, Weblogs.com simply shows the most recent blog updates automatically as they happen.
Icerocket - a blog search engine that also keeps track of blog statistics with their Blog Tracker product.
blo.gs - a simple listing of recently updated blogs in the spirit of Weblogs.com.
blogdigger - blog search engine, also has a Local feature for finding bloggers in your area.
2rss.com - a categorized blog directory and search engine with about 10,000 feeds indexed.
WeBlogALot - a blog directory that also pulls in breaking news feeds from mainstream sources.

RSS tips & hacks

Steve Rubel’s collection of RSS hacks
Another set of RSS-related hacks from Steve Rubel
Web Worker Daily’s tips on RSS reading
5 RSS tips by LifeDev
Steve Rubel’s 35 ways to use RSS feeds
RSS tools for Firefox - a detailed roundup of RSS-related add-ons for Firefox here on Mashable
Mister Tipster - a hardware USB mini display aimed specifically at displaying RSS feeds
Some RSS tips from Wired
Read RSS on your AppleTV

Miscellaneous RSS-related tools

SearchFeedr - RSS feed search, supports Yahoo!, Google, MSN Live and Altavista
FeedCycle - bundles feeds into “episodes” and delivers them on a scheduled/daily basis
FeedCrier - brings RSS feeds to your instant messenger
Clicky’s RSS feed - Clicky is a web analytics tool which can deliver analytics results via an RSS feed
RSS Panel X - a Greasemonkey script that displays RSS, Atom, hAtom and OPML directly from originating website
FeedForAll - create and edit RSS feeds and podcasts
Radio UserLand - a combination of a publishing platform and a news aggregator
Custom Reader - a white label RSS reader solution, ready to be branded to your liking
Chaos Wallpaper - RSS reader and wallpaper changer in one
FireAnt - RSS reader focused on podcasts, vidcasts, and media
NewsAloud - converts RSS feeds to voice
RSS2PDF - converts RSS feeds to PDF format
Dapper - creates an RSS feed from any website
Feedity - another RSS generator that can generate an RSS feed from any website
RSSMicro - an RSS search engine; searches over 65 million feeds