Here is a list of 65 Math books available online for free, compiled by George Cain of Georgia Institute of Technology. Including really interesting ones such as:
[Via reddit.com: programming]
Also, "comprehensive mathematics for computer scientists" - very good printed book, with a nice online version: http://www.ifi.unizh.ch/math/bmwcs/
-they have live examples using Java. finally a use for applets.