Dune - Frank Herbert Hyperion - Dan Simmons Cryptonomicon - Neal Stephenson Peter F. Hamilton's 'space operas' are pretty good too. I'm currently reading the stories of Jorge Luis Borges, whist not strictly SF, they are pretty weird.
I haven't used anything other than the Amazon one myself (I usually get pretty good recommendations there). If I am looking for something new to read, I usually go to an internet top 100 list and find something that looks interesting. Something like this one. http://home.austarnet.com.au/petersykes/topscifi/lists_books_rank1.html There is an older one I used to use all the time, but I can't find a link to it. There are other engines on the internet to recommend books like this. http://www.whatshouldireadnext.com/
Neal Stephenson - Anathem, Cryptonomicon Greg Egan - Diaspora, Schild's Ladder Ian M Banks - Excession For anyone with an interest in where modern physics is, a superb (and very accessible) text is The New Physics For the 21st Century, edited by Gordon Fraser Cambridge UP