I'm in ProbStat this semester. I didn't feel like working so I wrote a quick little program.
Tools like SurfStat are nice, but they aren't quite what I want.
So, I wrote a similar web app, Improbability. You can tab through the fields more easily and simply press Return to evaluate. Also, there is a graphical display of the area under the curve (using CSS and transparency).
So far, just the normal distribution until I'm satisfied with the interface, but other distributions will be pretty easy to add. I just wrote this, and am open to suggestions as to making it better.