Right now Subversion is very popular, having taken the reigns from CVS. However, Git is coming up fast particularly in the Ruby community. If you've got money to spend, something like Clearcase might be the way to go -- the others I list are free.
Source(s):
http://subversion.tigris.org
http://git-scm.com
http://www-01.ibm.com/software/awdtools/clearcase/

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Git is the new fast-rising star of version control systems. Initially developed by Linux kernel creator Linus Torvalds, Git has recently taken the Web development community by storm. Git offers a much different type of version control in that it’s a distributed version control system. With a distributed version control system, there isn’t one centralized code base to pull the code from. Different branches hold different parts of the code. Other version control systems, such as SVN and CVS, use centralized version control, meaning that only one master copy of the software is used.

Here's an awesome site I found with reviews of the top 7 source control systems:
http://www.smashingmagazine.com/2008/09/18/the-top-7-open-source-version-control-systems/
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