Active GPS Tracking
Active GPS tracking means you can view the location of the GPS unit from a website at any time. Do not get a 'passive' GPS tracker because you have to physically remove the GPS unit from the car and plug it into a computer to determine where the car has been.
OnStar - Provides a subscription based service that includes the ability to pin point the location of your vehicle via GPS.
Track What MattersTM - They offer GSM-based fleet tracking or satellite tracking services.