Adobe has a very good ePub reader, called 'Adobe Digital Editions'[2] that runs on Mac, Windows and (I assume) Linux. It's an Adobe Flash/Air application and is much prettier than Calibre. If you don't mind Flash on your mac (I gave up last year), It's currently the best choice.
[1]: http://calibre-ebook.com/
[2]: http://www.adobe.com/products/digitaleditions/
[edit] No, there's no Linux version. You can use it with WINE though; use the instructions here: http://ubuntuforums.org/showthread.php?t=701191
Use Calibre (right click -> tweak ePub -> explode ePub), or eCub[1] to see and manipulate what's inside an epub...
However, be warned that it won't allow you to copy text even from an non-DRMed ebook, which is quite silly and makes it useless for anything that has code samples in it. For reading a book on screen it's pretty good though.
[1] http://www.amazon.com/gp/feature.html?ie=UTF8&docId=1000...