I must second the Baen books site. I have repeatedly asked for a webscription for birthday and christmas and ect. Darn!! I guess I will have to get it for myself. (None of my family understands or likes to read e-books but me, they all want only dead tree style reads ) Another source is to check for newsgroups related to e-books. That is where I found all of the sites listed above.
I have this horrible addiction to reading SF

and it just seems appropriate to read it on a Z :!:
V-Man