Version control systems allow you to keep track of old file versions. This is particularly useful when finding bugs in software, or keeping track of changes to documents.
See also (distributed) | :bzr :git :fossil :hg :vv
See also (centralized) | :svn
See also (clients) | :rabbitvcs :tortoisegit :tortoisesvn
See also (other) | :be :myrepos
Ethical criteria for forges