If you like to support open standards format, here is a (at least for me) new discovery: .epub.
If you like open standards and ebooks, support the spreading of .epub. This file extension is a mixture of three open standards OPS ( Open Publication Structure ), OPF ( Open Packaging Format) and OCF (Open Container Format), produced by the IDPF.
From what I know Kindle – the hot ebook reader of the moment – does not support .epub format However, you can find some .epub ebooks in various websites, including Project Gutenberg. Viewing these files will be no problem with a software like FBReader.