OpenWithTest is a Visual Studio 2010 extension which serves one simple task: To always open your test files and implementation files together.
When writing unit tested applications (especially while practicing TDD) you will often open an implementation file (i.e SomeClass.cs) followed by the test file(i.e. SomeClassTests.cs). This extension makes this a one step process.
It works by detecting when you open a new file and attempting to find via convention the test file. It assumes that you create one test file per class. So, if you create a class called Car in the file Car.cs then you will have a test file named CarTests.cs which tests the car class. Currently, only C# (.cs) files are support but I plan to expand this to other files types soon.