Red Gate recently introduced .NET Demon, a new extension that continuously tests and compiles code in the background in Visual Studio.
In order to support continuous compilation, .NET Demon replaces Visual Studio's build system. To make this process faster, it detects and only compiles the portions of the application that have changed, or are affected by changes. The tool also continuously saves code as the developer types.
For those using NUnit 2.5.4 or higher for unit testing, .NET Demon adds continuous testing features to Visual Studio as well. As changes are made in the code, unit test results and code coverage are displayed inline. (All other features of .NET Demon are available whether or not unit testing is being used.)
.NET Demon works with all editions of Visual Studio 2010 except Express. While the full version of the software will be paid, the beta is currently available for free for download from Red Gate's website or the Visual Studio Gallery.