Post your favorite sources of e-books here as replies to this thread. Please indicate whether the site is free or pay (or mixed), plus an indication of what the site is like and what kinds of books can be found there.
I'll get things started with my favorites, which are some of the heavy hitters that should be bookmarked by every PDA owner:
1. Project Gutenberg
[Free] This is a huge collection of classic texts that is sure to include your favorites. It is growing all the time, and the most popular books are even available on a CD or DVD that you can receive for free if you don't have adequate online access. You might even want to consider donating a small amount to support their ongoing work to expand their library. And don't miss their page on converting texts
for PDAs and eReaders
[Free] Another giant collection of texts. This one is not just classic texts, but does include classics. The advantage of this site is that you can usually find a PalmDoc or iSilo text for most of the books. Other PDA formats are also supported on some books. They have their own excellent guides
for many aspects of ebook reading on PDAs. It's a great resource even if you don't download any of their books.
[Mostly Pay] This site is a bookstore with DRM-protected books designed to be read on their own eReader software, available on platforms including Pocket PC and Palm. The have a few free books here also. Commercial books are "ready to go" and usually are formatted a bit nicer. Most modern books and best sellers can be found at online bookstores, but not at the major free sites.
[Mostly Pay] Another excellent bookstore with DRM-protected books. This store offers books in various formats, including secure versions for MobiPocket, MicrosoftReader, AdobeReader, PalmReader and FictionwiseReader. They also offer many books in multiformat, which are not DRM-encrypted, and include up to eight different formats including Adobe Acrobat (PDF), Mobipocket, Palm Doc, iSilo, Microsoft Reader, Franklin eBookMan, Hiebook, and Rocket (REB/1100).
5. Christian Classics Ethereal Library
[Free] This is a bit more of a specialty site, but an excellent one with wide appeal. It provides a massive collection of texts, making it an unbelievable library of the classic Christian writings, covering a wide range of authors. A delightful source because most of the best Christian writing was done a long time ago, and therefore is copyright-free. [Edit: This link may not be available for a while. The CCEL editor forums indicate that the server has been crashing the last few days. May take as much as several weeks to repair. Seems to be run by a Professor, probably at Wheaton College, and this is a busy time of the semester for him. I'm sure it will be back, better than new, in the near future.
With the growth of online book collections and ebook reading devices such as PDAs, this is truly a great time to be a book reader!
Be sure to add your own favorite sources of ebooks. There are many more out there, and the sites above are just the tip of the iceberg!