Subversion

A List Apart has an article about using Subversion to help with collaboration, especially with contract workers. Though there are some superior version control systems mentioned in the comments, like git, mercurial, or bazaar, they don not have the client support that Subversion has as of now. I especially use the Subclipse plugin for Eclipse for my work and checking code in and out of Subversion is a quick right-click away.

More importantly, how much client support for Subversion there is makes it easier to get other developers on board using itYes, most developers I work with try to do as little as possible on the command line....

At work, I've setup a Subversion server for the several developers to use while coding for our current and future projects. The other developers used Windows for their O.S., so they are able to use TortoiseSVN to check out, commit, and other functions on the Subversion repositories.