Better SCM Initiative : Perforce
Note about Relevance
As of 2018, this site has become mostly irrelevant.
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Summary
Perforce is a centralized, commercial (non-free) solution for version control. It is very fast, very stable and robust, scales very well, and has a good reputation. It requires an annual per-seat licensing, but is also available for interested open-source developers under a gratis license.
(If you're looking for something open-source which is more or less equivalent, then look at Subversion.)