From Perforce Software:
Perforce, the Fast Software Configuration Management (SCM) System, provides powerful version control and workspace management with a unique branching model. Perforce provides software developers with fast and reliable SCM to manage source, document and Web content. The client/server architecture scales to hundreds of users and millions of codelines. Perforce works well even on limited bandwidth networks. The server's repository sits on a central host; client workspaces can be anywhere on the network -- around the office or the world.
Atomic change transactions ensure access to a consistent repository state at any point in time. Inter-File Branching helps to manage multiple development lines and easily integrate changes between branches. Perforce's self-maintaining and reliable database keeps data safe.
The command-line client is 100% scriptable, 100% portable and available on more than 50 platforms. The Windows GUI and browser-based clients provide a user interface for all SCM tasks. The Perforce Reporting System tracks metrics like the number of changes in a release or who has files open for edit. Perforce also provides basic built-in defect tracking, is integrated with several leading defect tracking systems, and includes a kit that allows users to integrate Perforce with the defect tracker of their choice.