We present Matchbox, a toy race-condition finder — it's an analyzer for a toy assembly-like language that runs in a web browser. It finds all interleavings between two programs, runs them all on shared memory, and declares there to be no race conditions if and only if every interleaving is either impossible, or produces the same result.
I wrote Matchbox to (cheesy as it sounds) raise awareness of race conditions. Also, Peterson's Algorithm is beautiful, and Matchbox graphically demonstrates that it does work. For a fuller account of why I wrote Matchbox, see the commentary on the Matchbox node, and the projects' README file.