Check it out: http://www.davyw.com/quantum

For a good "Hello world" circuit, here's the diagram for a Bell State...

Try evaluating it yourself and see that |00> or |11> are the only possible outcomes.

You can even compile your own circuits into gates to use in other circuits.

Construct a circuit that transforms a single qubit into |0> with 75% probability and |1> with 25% probability, such as:

Then compile that into gate "F" and use it to create a superposition of |00> with 75% prob and |11> with 25% prob, like this:

Let me know if you find any use out of this. And definitely let me know if you find any bugs.