TestDriven.NET is a popular plug-in for Visual Studio .NET (formerly known as NUnitAddIn) that allows you to easily run your unit tests within the IDE.
Some of my favorite features include:
Ability to run your test on a single method, within a class, or within a namespace
Jump into the debugger by defining breakpoints within your test fixtures
Convenience of not having to switch to an external program to run my unit tests (nunit gui, etc.)
Failing tests show up in your Task List and are just a double-click away
If you're Test Driven and want to try an add-in that I feel will positively boost your TDD experience, check out TestDriven.NET!