[TIPS] Four math sites 4 U

Someone recently asked for some new math sites. Here are three that I know I haven't done before, and the last one I think I HAVE given you before. All came from browsing through the Math tag at del.icio.us.

http://home.swiftdsl.com.au/~mattg/

http://www.figurethis.org/teacher_corner.htm

http://archives.math.utk.edu/visual.calculus/

http://integrals.wolfram.com/index.jsp