100 cab fares unlocks new fast cab called the Borgnine, in the Harwood area.
50 ambulance missions you get health and power-ups at your hideout, 100 will give you an adrenaline pill.
30 fire engine missions in each island thats 90 in all, will unlock the flame thrower in your hideout.
40 vigilante missions in each island thats 120 in all. will unlock 2 police bribes at your hideout.
These are all taken from a magazine and I haven't tried them so theres no guarantee they will work.