Cervisa (Linux/Unix)

For *nix, one possibility is Cervisia. It's a KDE application and will integrate with Konqueror, but I prefer it stand alone.
>>Cervisia screenshot, showing branches on drupal's CHANGELOG.txt

You can configure it to use "kompare" for diff:
>>Kompare screenshot (CHANGELOG.txt)

Here are links for more information about Cervisia:
- http://cervisia.kde.org/
- http://docs.kde.org/stable/en/kdesdk/cervisia/index.html
- http://cervisia.sourceforge.net/documentation/index.html