SVN Time-Lapse View is a cross-platform viewer that downloads all revisions of a file and lets you scroll through them by dragging a slider. As you scroll, you are shown a visua
It's not quite a slider, but the Git bundle for TextMate lets you browse revisions for a single file via a dropdown menu. It highlights changes for the current revision, and also names the person who last edited a line, svn blame
-style. There are also keyboard shortcuts for navigation to previous/next revisions.
http://blog.macromates.com/2008/git-bundle/
Update 2014-05-10: Now available on GitHub: https://github.com/textmate/git.tmbundle