If you steal or hijack a taxi you can press Num + or Caps Lock (R3 for the PS2 and X for the X-Box) to activate bonus Taxi missions. It allows you to pick up passengers and drive them to their requested destinations. The faster they get there, the more money you will make. You can recognize the passengers by the blue arrow over their heads.

This is a great way to get to know the streets which will help you when you do missions or need to get away from the cops. If you drive 100 passengers you will unlock the Borgnine Taxi which can be found at Borgnine Taxi's parking lot in Portland. You dont have to do all 100 drives at the same time, you can do them whenever you want to.

The Borgnine's location is marked on the locations map with this icon: