A really really good source of free eBooks for the Sony Reader line (500/505), Kindle, and any other device that reads Mobipocket format is MobileRead http://www.mobileread.com/forums/
because people there have been posting lots of public domain eBooks and freely available eBooks. A lot nicer formatted then the ones Manybooks gives you. There is even one Star Trek eBook available for free.
Good to know JWolf
, I'm always happy to find new sources of reading material for the Kindle, especially if they're free.