A German firm has developed a digital pen that drivers can use to pay for the MAUT (German Motorway Toll). You can get information at either of these two sites, although they are both in German.
You can also download a pdf flyer (2.76 MB) in English here
The driver uses the pen to enter their route on a special form and the information is then sent automatically, via a Nokia mobile, to Toll Collect and the driver will receive an SMS with confirmation of the trip and that the toll has been paid.
The company claim that in contrast to other solutions such as an On Board unit or the roadside machines the Digipen has low overheads and is easy to use.
The cost of a Digipen MAUT connection kit is around €500 which is getting on for twice what it costs to have an OBU installed. The Digipen is not tied to one vehicle which makes it more flexible over the OBU.