hacker.org

A chinese site (pages are written in English btw)
http://read.freeduan.com/
this is the best site i know where you can find ebooks on almost every science (the math and qm sections have very good collections)

thanks i have been looking for a good read on quantum mechanics

the best place for ebooks is mininova.org section "Books"... you can find anything from Chemistry, Physics, Biology to Programming, IT Security, Mathematics and so on...

... a while ago I tried a pair of math e-book collections on mininova, they were full of trash, lacking masterpieces and with some wrongly categorized material (just what you need to guess how much the one who did the collection understood), but maybe the situation with IT and other sciences is different