I’ve added in support for drawing your own boxes (as many as you want!) into gwt-phys2d. It’s fun to try to do things like create a long see-saw shape and try to balance boxes on both sides. Check it out: http://gwt-phys2d.appspot.com/
If you try drawing enough boxes, you’ll find that performance doesn’t scale all too well (or at least it doesn’t on my netbook). I watched memory in the task manager and saw it repeatedly spike and fall back down, suggesting that it’s definitely the garbage collector which is eating up much of the time. I still haven’t had a chance to rewrite the phys2d code to stop creating so many new objects (esp. collision Arbiters) every frame, but I imagine that’ll improve performance considerably.
I’ve also created a Google Code and subversion repository for the project: http://code.google.com/p/gwt-phys2d/
If you’re interested in contributing let me know, and I’ll be happy to add you. Here’s a screen capture of the current version: