Wolfram|Alpha
I just ran into the Wolfram|Alpha site (found it via NyTeknik). The site is amazing. Try, for instance, to search for a substance: caffeine. The result not only includes a nice picture of the molecule, but also quite a few facts about the substance.
Looking a molecules is fun, but the man behind W|A was also responsible for Mathematica. So, go ahead, enter a formula: sin(x)^2. The result includes period, a plot, derivate, integral, but also the steps taken to reach the results. I can imagine one or two maths students using this.
Well, sorry for bugging you with this - but I was absolutely stunned when I ran into the site. It has earned itself one of my bookmarks!