calibre can convert from a huge number of formats to a huge number of formats. It supports all the major e-book formats. The full list of formats can be found here.
The conversion engine has lots of powerful features. It can rescale all font sizes, ensuring the output e-book is readable no matter what font sizes the input document uses. It can automatically detect/create book structure, like chapters and Table of Contents. It can insert the book metadata into a "Book Jacket" at the start of the book.
Re: Calibre (eReader / eBook Converter) I've known about Calibre, and I have it installed. Many people use it to create backups for the ebooks they bought. (Technically, you only buy a license for an ebook, not the book itself...)
I've also been warned against using Calibre to create mobi-files for the Kindle. The code it creates is said to be not clean and full of mistakes. I cannot verify that, but that's what my formatting people said. (Who have an interest of keeping me as client.)
Re: Calibre (eReader / eBook Converter) A book on self-publishing that I have recommends Calibre. Claims it works pretty well. (I was only skimming, so I'll share more info once I get into reading it more thoroughly. I'm also doing some searches for the best free ebook converters.) The book also recommends using Scribus for creating both the book and the covers/jackets. I may do that too.