Wolfram|Alpha

Looks nice from a first insight :D

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 (Thinking)