I don't know if this app has already been mentioned before, but I thought I would say something since we're leaving Rome today.
My husband downloaded the "Roma Bus" app for his iphone, and it is awesome. You can type in the bus stop or the bus number and it will bring up where the bus currently is or what buses come to that stop. You can see a list of all the stops for the bus, etc. For example, we were waiting for the 87 bus last night and we watched it approach, stop by stop, on the app. Of course you have to have internet service to use it (we rented a mifi from xcom), but it was really helpful.