Help: ePub

The ePub (2007) file format is a standard for e-books supported by the IDPF.
ePub is actually divided into 3 different standards: Open Container Format (OCF), Open Packaging Format (OPF) & Open Publication Structure (OPS).
It is a natural evolution of an older standard, OEB (1999), which is at the core of various formats (Mobipocket, LIT etc...).
ePub is a reflowable format, which means that unlike PDF, a single file can adapt itself to any device, any screen.
Compared to similar reflowable formats, ePub is much more advanced:

• Full CSS support
• Embedded fonts
• Support for a real table of contents (NCX)
• A powerful fallback mechanism

Overall, if your reading system can support ePub, it is highly recommended to use this format.
Feedbooks is probably the first platform who fully supported ePub, and all of our files (books, RSS feeds) are available in this format.
We won't go through all of the reading systems that support ePub, but here's the list of the most noteworthy:

• Adobe Digital Editions
• Stanza
• FBReader

Adobe Digital Editions is a free desktop software available for Windows and Mac OS X.
It can handle almost all of the ePub specs (unlike the rest of the applications on our list) and provides a rich reading experience. Through it's library feature, Digital Editions supports mobile devices too, such as the Sony PRS-505.

Stanza by Lexcycle is available on both Mac OS X and the iPhone/iPod Touch.
Feedbooks is fully integrated into Stanza's catalog system.

Available on many different platforms, FBReader provides a basic support for ePub. Originally written in C++, FBReader is currently ported to Java (screenshots on Android).