Normally version control systems don't allow fine grained commits. commit-patch allows the user to control exactly what gets committed by letting the user supply a patch to be committed rather than using the files in the current working directory.

commit-patch supports Darcs, Git, Mercurial, Bazaar, Monotone, Subversion, or CVS repositories.

Also included is an Emacs interface to commit-patch. It allows you to just hit C-c C-c in any patch buffer to apply and commit only the changes indicated by the patch, regardless of the changes in your working directory.