(The list is also available in German: RSS-Reader Übersicht)
These are the most popular RSS readers for Windows, Linux, MAC, email and web-based readers, reader for mobile use and clearly laid out in a list.
This free and non-commercial site were last checked on June 27th 2012.
Windows RSS Reader 
Alnera FeedBuster 
Free RSS reader with many features. Ideal for beginners with no programming knowledge.
Awasu 
Free RSS reader with many features.
FeedReader
One of the most popular German Reader (Version 3.0).
FeedDemon
One of the most popular RSS readers for Windows.
Fuzzy Duck Reader
Free RSS, RDF, Atom Reader.
KlipFolio
Desktop client for Windows for each RSS feed.
NewzCrawler
RSS- und News-Reader, Browser und Blog-Client in einem.
Twins Webnews
Clearly constructed RSS-Reader.
Omea
Free Windows RSS-Reader and news aggregator with many features.
RSSBandit
An RSS-Reader works with google-reader and facebook.
RssReader
Simple Windows RSS Reader. Last updat 2007.
Windows .NET-Framework
intraVnews
RSS news aggregator for Microsoft Outlook.
RSS Bandit
Free. NET RSS aggregator for your desktop in Windows.
SharpReader
SharpReader is ein.NET-based readers with the most important features, OPML export, and the first reader of the wfw: commentRSS support element.
Mac RSS Reader
NetNewsWire
RSS-Reader for Mac OS X.
NewsFan
Mac OS X aggregator with Watchlist, category sorting, built-in “translation feature.”
Newsfire
Simple but popular RSS reader.
NewsLife
Pay RSS reader for 8 €. Recommended!
Shrook
Fast, easy reader. Can be synchronized with mobile devices, Mac OS X. The reader is shareware, which can be tested 30 days.
Vienna RSS
Free RSS reader. Structure as an e-mail program. Highly recommended.
RSS-Reader als Dashboard Widgets
Free reader.
Linux RSS-Reader
blam!
Simple feature-rich Linux RSS reader.
Liferea
RSS reader with support for Atom-GNOME. Many features and easy operation.
Snownews
Free text mode RSS reader for Linux, Mac OS X and Solaris. GPL licensed.
Straw
Free written in Python desktop RSS and Atom aggregator for GNOME 2.
Syndigator
Gtk-based RSS reader for Linux.
Raggle
Linux RSS reader based on Ruby.
rawdog
Linux simple RSS reader. However, was not updated for a long time.
Rol
Recommendable Linux reader.
Mobile RSS-Reader
Google Reader Mobile
One of the most popular RSS readers for mobile phones.
Lite Feeds
Feed Reader for BlackBerry, Java Mobile, smartphones running Windows Mobile. Online registration is required.
Opera Mini 4.1
With the mobile web browser Opera Mini 4.1 is a free application feed reader is integrated with interesting features.
Yahoo! Mobile
Free service from Yahoo. Yahoo Mobile: RSS reader for mobile version.
Web-basierte RSS-Reader
Bloglines
Free, web-based reader and aggregator service.
freshnews
Free Online RSS Reader for feeds in RSS, RDF and Atom. Simple and convenient online reader. Recommended!
Google Reader
Very simple and packed with features versehner Online Reader. Highly recommended!
Web-besed Clients
Feed on Feeds
In PHP, MySQL and the MagpieRSS class-based scripts. Just copy it to your own server and off you go.
zFeeder
PHP-based reader which allows you to integrate feeds into web pages.
Cross-Plattform
AmphetaDesk
Is one of the most popular free RSS readers out there right now. The open source solution includes many features and very extensive multilanguage documentaries for developers.
Blogbridge
Free open source reader for Windows and Mac.
BottomFeeder
Free open source reader for Linux, Mac and Windows with a large feature list.
Pears
A free on Python-based RSS reader. Pears can be run from a USB flash drive, and provides support for OPML. Is under the GPL.
RSSOwl
RSSOwl is a Java-based Reader with built-in browser, OPML and PDF export, and categorization. It is available in different languages.