The Kindle can read any non-DRMed Mobi files (which are available from places like Baen, and Books On Board, I think. I've only used Baen so far), plus it's own DRMed Mobi files (.azw and .azw1) and TXT files. Non-DRMed files in other formats like ePub can be converted to regular Mobi and loaded onto a Kindle; you can find instructions for it by poking around Mobile Read. You can find instructions for converting just about anything into just about anything else around here.
I've been meaning to play around with Calibre and convert some of my Google Books ePub files to Mobi but I haven't gotten around to it yet. So many eReaders, so little time....