Note: There's no separate version for classic. This should work fine in both retail and classic.
ProgressBar is a very small and simple stand alone AddOn that displays your experience/reputation in the otherwise empty "name bar" of the standard player frame. Nice to have if you, like me, are using the standard Blizzard player/target frame but don't want use the standard Blizzard actionbar and experience bar.
ProgressBar shows your experience progress by default but if you check the "Show as Experience Bar" for a faction under the "Reputation" tab then the ProgressBar will instead show your reputation progress for this faction.
When showing experience the color of the ProgressBar is blue/red (blue for alliance, red for horde) and the amount of rested experience is shown as a purple/dark red bar. When showing faction reputation the color of the ProgressBar changes according to the current faction standing (same color coding as you would find on armory).
Left/right clicking the ProgressBar cycles through four different display modes:
1. Character name / faction name.
2. Percent completed of the current level/standing.
3. Amount completed / max value of the current level/standing.
4. Amount remaining to the next level/standing.