Jim Radford and David Caldwell wrote a neat little utility that lets you check
in select portions of a file into Darcs, Git, CVS, Subversion, or Mercurial. It
comes as a command line app and also and emacs interface.

A blog entry (http://porkrind.org/missives/commit-patch-managing-your-mess)
about commit-patch provides some detailed information on why you might want to
use commit-patch.

commit-patch and commit-partial can make commiting partial changes easier with:
-darcs
-git
-mercurial
-cvs
-svn

commit-patch-buffer.el 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.