Wolfram|Alpha
Looks nice from a first insight
And I think it provides some features from Mathematica. You can compute (in)definite integrals, multiple integrals, derivatives, plot graphs, find the power series representation of a function, recognition of sequences, etc...
And among the useless things : see how long (in days) the world has been going well thanks to your presence.
The drawback of this, imo, is that people will become more lazy
This is a really neat project in its early stages.
If you haven't already, watch the overview video/screencast:
Introduction to Wolfram|Alpha by Stephen Wolfram
It lists some really cool examples of what you can do or look up.
Recently the 2 resources I have been using most consistently in my classroom are Wolfram Alpha: Wolfram|Alpha and Game Classroom math games: Math Games
My students love typing in their names into Wolfram Alpha. Have you tried?