Wolframalpha

If you haven't seen wolframalpha yet, it's worth checking out. Some people are trying to play it up as a competitor with google. If that's how you view it you'll be very disappointed. If you take a look at their sample searches there's some cool stuff there. But what they don't seem to play up is that you now basically have free access to mathmatica (one line at a time).

So try stuff like
10*x^3+0.5*x^2+x+100=0
or simplify (x^2 + 4*x + 3)
or d(e^(x^x))/dx
or 4 * (boltzmann's constant) * (300 K) * (100 ohms). Yes, google calculator will do the last one, but it doesn't change the units to (nV^2)/Hz like wolframalpha does.