In Visual Studio, I use Ctrl+F12 to go to the implementation of a method. Just now, I accidentally pressed Alt+F12 and what happened next blew my mind. An inner editor window opened under the line of code I was on, with the code for the method I wanted to see visible.
I have no idea what this feature is called, or whether it is from Visual Studio or Resharper, but I'll be using it a lot, especially with test-driven development.
Update: I just googled it, and it's called Peek Definition.