Websites... not really. You can sometimes find big packs of mathematics text books on the torrent sites.
Here is a list of books online. Oh, and the AMS had a series of free books in PDF format.
http://people.math.gatech.edu/~cain/...linebooks.html
Wikipedia is surprisingly good (especially in breadth), but its depth is limited.
I suggest getting Dover editions of math text books. Dover republishes older books and sells them at good prices. Also, Alibris.com is a good resource for used text books. I have bought books in good condition for as little as 99c. I can write up a list of good books if you want.