Windows/Mac/Linux: Calibre, one of the best free ebook managers out there (and a great tool for removing DRM), released a big update this week. Calibre 2.0 includes a new editor function, comparison tool and support for Android phones on OS X.
The big new addition here is the editor. You can now edit ebooks in a lot of ways and export them to EPUB and AZW3 formats. Also new is the ability to compare two books so you can look at edits side-by-side. A few other small improvements are in this update as well. For instance, if you're an Android user who's also on OS X, Calibre finally works for you, and a number of improvements to the library organisation make it a bit easier to use overall.
Calibre 2.0 (free)