Mighty-Moose, a continuous build and test-runner for .NET, now comes free of charge.
What exactly does Mighty-Moose do? Every time you hit save after editing a file in Visual Studio, it automatically builds your solution and runs the tests that get affected by that change. This is great for TDD because you no longer have to run the tests manually and get instantaneous feedback, without leaving the code editor. This also save time that would otherwise be spent running tests that are not really affected by the immediate changes.
There are other configurations as well (such as run all tests after running impacted tests first, or run on every build instead of every save) but the above is expected to be the main use case. The tool also has several other helpful features listed on the project website.
Part of Mighty-Moose is open source in the form of AutoTest.Net – this has the same continuous building and testing features. However, unlike Mighty-Moose, AutoTest.Net is not selective about the tests it runs. Both the projects are developed by Greg Young and Sven Ackenhausen.
As to why this tool is free when it could be licensed for a fee, Greg explains.
NCrunch, which is currently free to use in beta, is another tool that can run tests continuously. NCrunch focuses on prioritization of the tests and performance through parallel test execution.