I'm sure you already know about Baen's Free library. I always knew about it, but could never get myself to read a whole book on a browser. Well, and this is probably something you already know as well, but it's also really easy to download any of their free library directly onto your eReader.
Here are a few of the free books I just downloaded:
There Will Be Dragons by John Ringo (Always wanted to read this, but this cover's bad even for Baen)
On Basilisk Station by David Weber
Sheepfarmer's Daughter by Elizabeth Moon
1632 by Eric Flint
There are also plenty of other high profile authors I haven't read, but that I'll be checking out such as Mercedes Lackey, Lois McMaster Bujold, and David Drake. Thought you might want to know if you didn't.