A good place for free ebooks is your local library. Check to see if your library loans out ebooks (click Libraries tab and enter your zip code). With just about any format except Amazon Kindle's proprietary format, your library holds many new titles. My branch is very busy though, so most books require waiting periods. The process is the same as signing them out normally, although you may need to download and install Adobe's Digital Editions to authenticate the DRM'ed ebooks.
For most of the Western literary canon, you have Gutenberg.org. They digitize out of copyright books in multiple formats. This includes all those pre-1920s classics you read in high school English class. Gutenberg also has a large selection of 1950s sci-fi which, due to lack of copyright renewal, are now public domain.
You can find modern sci-fi and fantasy books at the Baen free library. Annother source of Baen free ebooks is the Fifth Imperium. Baen often includes CDs in the later books of their series. The CDs often include the entire series and Baen allows legal distributing and downloading of the contents. Their strategy of hooking readers on the free books so they buy more books worked for me at least. Sheepherder's Daughter(the download options at left contain the entire book, the sample chapters link do not) was a trilogy and I bought the combined trilogy omnibus paperback after reading the first one.
I recommend the Belisarius series by David Drake and Eric Flint (right hand column of the linked website) and the Miles Vorkosigan series by Lois McMaster Bujold.
A much smaller library of books are available at Suvudu.
A great blog highlighting book deals is Books on the Knob. Amazon Kindle and then Barnes & Noble Nook release the most free books but often I find cheap or free deals on books compatible with my Sony reader. The blog updates often, so check it out!
A forum I browse for free ebooks is Mobile Read. Besdies collecting the latest news on ebooks, you can get reviews and opinions on ebook technology, hardware, and plain old discussion on the stories in the books themselves. The free ebooks forum is great for finding deals, coupons, and free, legal downloads.
Manybooks.net describes itself well. I like Lucky Stiff, an old pulp novel about a woman framed for murder, only to escape from the electric chair by a lucky last minute reprieve. She then heads out to get payback on the people who framed her.
Munseys is an unusual site listing many old pulp novels.
Below are a few other sites I found from a Woot.com post that I haven't checked out yet:
http://www.fictionwise.com/ebooks/freebooks.htm
http://www.feedbooks.com/
http://inkmesh.com/free-ebooks/
The Kindle today announced they are selling ad-supported versions of the $139 Wireless Kindle (no 3g) for $114. Amazon is dangling ads for deals like a $20 Amazon card for $10 and an mp3 album for $1. I'm in! (the $114 version is inder the Kindle with Special Offers box at the Amazon product page)
Happy Reading!