Our programming mentor's husband works for Microsoft, so we got Visual SourceSafe. Beautiful peace of software (well, decent. Just sucking up becuase he told me he could get me a beta version of Visual Studio 2005.
).
We had two programmers working, so it was very helpful in cooridating work, especially on the utility functions which we were codependent on. (I wrote autonomous and some utility stuff for motors and he wrote the driver control and encoders.)