Simulators are in fact programs that make some little guys juggle on a computer screen. When it comes to passing correctly, there are not a lot of them (would that be like in real life ?), and the winners are JoePass! and Jongl.
JoePass! is more used throughout the site, but that's only because I discovered it before Jongl. I recommend trying both of them. JoePass! has some very convenient features for creating your patterns while Jongl offers some nice output on the screen.

An overview of JoePass!	An overview of Jongl

You can also have a look at what Itsik Orr wrote about the 2 simulators:
Itsik Orr's review of JoePass!
Itsik Orr compares JoePass! and Jongl