Try out FactionFriend by Fizzwidget. If you are using the default rep bar or a mod that uses the default rep bar graphics, it will also display a tooltip while hovering the repbar showing how your rep will gain based on items in your inventory. It will also show a "Rested" type indicator on the bar showing how far your rep will go when you turn in what you have. It is a great mod. Of course it also shows in the item tooltips what rep the items are used for. Check it out at Fizz's site:
Faction Friend