CVS

CVS (Concurrent versions system) is a software development versioning tool. It is available for many platforms and is widely used in the open source and commercial software development industries.

You might also be interested in these highly recommended CVS resources:

Open Source Development with CVS by Moshe Bar and Karl Fogel
Pragmatic Version Control Using CVS by Dave Thomas and Andy Hunt
Essential CVS by Jennifer Vesperman

TechBytes in 'CVS'