I used Smashwords to publish through this medium. If you go to their website, you can submit your document (I think in HTML or Word form) and they will convert it. If you want to ship it into ebook stores (e.g. iBook Store, Kindle Store, Barns & Noble, and the Sony Store) you have to meet Smashwords style guidelines, but that's not too hard.
If you just want to convert files to different formats, there's freeware out there for that... I'll have to get on my other machine to see what I used.