ManyBooks.net has nearly 28,000 free e-books available for download. Each book is available in 23 different formats.
I wondered what software they used to create so many formats and how popular each format is. The site answers both these questions. This page lists the software used and this page [Update: page removed] lists the popularity statistics.
PDF is not as dominant as I would have expected. It is the most popular format, but it only accounts for 20% of downloads. Here’s a bar chart of the downloads per format. Click on the graph for a larger version.
See also Wikipedia’s comparison of e-book formats.