ThreadFusion is a new design aid to assist in developing multithreaded software. Mockup your multithreaded code, then simulate it's execution to verify your code is working as designed in ThreadFusion's visualizer.

ThreadFusion is a small desktop application that requires Windows XP SP2 or higher (Windows 7 preferred), .NET 3.5 SP1 or higher, and costs $99.99 US per license.

You can find ThreadFusion here:
http://www.chordfusion.com/

We've also put together a small Quickstart guide on how to get started using ThreadFusion, you can find it here:
http://www.chordfusion.com/quickstart.html