Pencil is a language for specifying Petri Nets that can interact with Java code. This was a one-term project in a graduate compilers class and as such exists somewhere in the zone between "good enough to get an A" and "good enough to actually use."
So you want to know more?
Now are you convinced? Download the Java source code. This includes lots of sample code, some of which actually works!
If you have any questions, contact cconway at cs dot nyu dot edu.Home