tkdiff (diff graphical front end)

tkdiff is a graphical front end to the diff program. It provides a
side-by-side view of the differences between two files, along with
several innovative features such as diff bookmarks and a graphical
map of differences for quick navigation.

tkdiff knows about AccuRev, BitKeeper, ClearCase, CVS, Git,
Mercurial, Perforce, PVCS, RCS, SCCS & Subversion.
The Git functionality requires that you prefix tags &c. with "-r",
unlike with git-diff.
The extensive online help is well worth reading.