The Baen Free Library. Tons of free books, and also an interesting essay on "piracy".
(Somebody once pointed out that calling file sharing "piracy" is trivializing the human tragedy of real piracy which is still a living business. I have a friend who experienced it as a child.)
... Hey, I just now see that good ol' Kathy "KD" Wentworth, who I went to writing workshop with in 1990, is represented here! Kool. She's a sweet and fun woman, and a good writer.
It took a couple of minutes for me to find the books in formats other than web pages, but it turns out that one can't find them via the "series" index or the "author" index, only through the "books" index (click link on the left). But then there are more formats than you can shake a Nook at.
Oddly, many of the book pages say "Ebook Price: $6.00", even though on the same page you can download the book for free.
Talking about free ebooks, check out Seth Godin's multi-author What Matters Now, a free PDF of super-short articles. It an unusual book and it's designed for easy screen reading in landscape format.
(You can even read it on the iPhone's high res screen if you have good eyes.)
(Tip: you can lock or unlock screen rotation on the iPhone by double-clicking the home button, swiping right, and clicking the circular lock icon.)