Algodoo

Its pretty good, but VERY easy to crack. If you know how these work you can:
know the code and unlock it
try to crack it.
So if you pay attention you should be able to crack it. Try before dissecting it!

INSTRUCTIONS:
Drag the numbers until the correct first number(left to right) is selected, then the next three numbers. it should unlock and a little U for unlock pops up, you can drag the hook thing out hook something I guess... drag it back, and then drag the U down and wait for the numbers to scramble. It is now relocked. This is just for show, I have seen better. Its just a proof of concept.