Spec Explorer

Recently Microsoft released Spec Explorer 2010. Spec Explorer is a tool for model-based testing. From the website:
Spec Explorer 2010 is a tool that extends Visual Studio for modeling software behavior, analyzing that behavior by graphical visualization, model checking; and generating standalone test code from models. Behavior is modeled in two ways: by writing rule machines in C# (with dynamic data-defined state spaces) and by defining scenarios as action patterns in a regular-expression style. One of Spec Explorer’s major features is the ability to compose models written in these two styles. This technique enables users to slice out test cases from large state machines by defining relevant scenarios, thus tackling the notorious state-space explosion problem pervasive in model-based testing. Spec Explorer also supports combinatorial interaction testing with a rich set of features.

The team that developed the tool also maintains a blog worth checking out.