Oh, I totally forgot to mention – if you’re looking for a quality EPUB reader, one of the better options is one built into a browser. Everybody has a browser, after all, and the quality of HTML5/CSS3/SVG1.1 support is usually top-notch… *MUCH* better than in your average standalone EPUB reader. In fact, even the better standalone EPUB readers basically use browsers internally. (Calibre, for example, doesn’t use FBReader, it uses its own EPUB viewer that’s based on WebKit (ie, basically Chromium/Chrome)).
Firefox has Lucifox, and Chromium/Chrome has Readium. Both are plenty good enough for reading, and both even offer limited library management if you want it (if you want full library management, there’s always Calibre). Both are free software – Lucifox is GPL3, Readium is BSD-licenced. The interesting thing about Readium is that it’s actually the reference implementation used by the IDPF (the people who write the EPUB specification) to show full EPUB 3 support.