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