Reputation Bars is a modular addon that lets you watch one or more faction standings, i.e. the amount of reputation you have with them. It can be used to track specific factions or automatically track the ones you gain rep with. It is especially useful for those with custom UIs that no longer have the default Blizzard rep bar available.
You can use this while leveling to see your reputation bar or simply as a way to show you how much you have left for various factions as you grind your way to exalted...
More information is available at WoWInterface