I use Centrafuse, which is a front-end software to run other software/plugins with a touchscreen (works with a mouse or touchpad, as well).
There's a plugin available for it, called CFFuelPrices, which uses your internet connection to get the fuel prices. You can look for prices on specific types of fuel (Unleaded, Plus, Supreme or Diesel) in the US, UK, France or Germany. You can look closest to you, based on the GPS location, or in a specific location.
Screenshot:
For more info on this specific plugin, see this link:
CFFuelPrices - Status : Update 12/06/10 - Centrafuse Auto - Connected Car Apps User Community
This plugin is free (he does take donations) and runs on the latest version of Centrafuse.
I've never tried this plugin, so I don't know exactly what all locations it pulls data for, such as truckstops or whatever. I can, however, load it up and give it a test drive if anyone wants me to check on it.