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.
calibre supports the conversion of many input formats to many output formats. It can convert every input format in the following list, to every output format.