Source Code Control (SCM) Tools
Lots of files (.bak, .old) – bad, not as easy as it appears. [free]
SCCS/RCS – suitable for single user, or pre-integration. [free]
CVS – open-source, teamwork, basic GUI available. [free]
Perforce [$600/user], SourceSafe [$500], PVCS [$570], ClearCase [$3000/seat], Sun Forte Enterprise [$2000] – high-end teamwork, rich GUI.