There's a hot new conversion site that just popped up in the news this week. It handles an impressive selection of ebook formats, and it's really worth a look. But what's even better is that it also converts audio, video, documents, and images. Check it out.
P.S. Most bloggers would end the post there. Not me. Are you looking to convert to a specific obscure ebook format? Then you probably want to visit the MobileRead Wiki. There are about 4 dozen different online conversion tools listed as well as over 100 other conversion tools.