These are the better cross platform RSS feed readers. Most of them run on Windows, Linux and Mac OS X.
RSSOwl lets you gather, organize, update, and store information from any compliant source in a convenient, easy to use interface, save selected information in various formats for offline viewing and sharing, and much more. It's easy to configure, available in many many languages and the best of all: It's platform-independent.
BottomFeeder is a news aggregator client (RSS and Atom) written in VisualWorks Smalltalk. BottomFeeder runs on x86 Linux (also FreeBSD), PowerPC linux, Sparc Linux, Windows (98/ME/NT/2000/XP), Mac OS8/9, Mac OS X, AIX, SGI Irix, Compaq UNIX, HP-UX, and Solaris. Features full support for CSS, including user defined CSS, synchronization of 2 or more BottomFeeders via HTTP or file import, plugins for blogging, IRC, and MSN Messenger contacts
BlogBridge is a feed reader that comes with a server-based service that will let you synchronize all your feeds across multiple computers, and even maintain your read/unread status across those computers.
Habari Xenu is a cool XUL news aggregator that is built on top of the powerful Mozilla web platform. It is an extension for the Mozilla and Mozilla Firefox web browsers.
Opera, the popular alternative browser, comes with an integrated feed reader.