right up on top of Max's list is Bartleby.com, which is an excellent resource.
a while back I got this note from a reader:
I wanted to let you know about ReadPrint.com
-- a massive non-profit library similar to Bartleby -- except its far better organized and user friendly. We've been using it extensively in school nowadays -- it's great for doing research since you can search within the books.
from a glance, it looks pretty good.