1. Interactive Math Miscellaney and Puzzles
Lots of cool math games and puzzles with Java applets that help you to figure out the answers and with solutions that explain how to figure them out. Try the eye opener series for some good challenges.
2. Virtual Bubble Wrap
I know this isn't particularly educational, but it is kind of fun. I suggest Manic Mode if you are feeling really crazy.
(This is just a picture, you have to go to the site to actually pop it.)
Two interesting links posted by Mrs. Davis each Tuesday during the school year.