Calibre (as always). (If you don't want to install a program to do book conversions, www.online-convert.com also works quite well to go back and forth from .mobi to .epub
FBReader (FBReader.org) is a solid ebook reader that handles non-DRM epub and mobi.
If you use the Firefox browser, you can also read using the Epub reader extension:
I have used all of the above (in Linux, no less) with great success and am very satisfied.