In Colombo - Kangaroo Cabs (especially when coming from the airport, only 1750! Book in advance, call when you arrive to avoid the "paging" fee)

In Galle - Red Cabs. Much cheaper than any tourist-gouging tuk tuk (but call early, they usually get booked up).

Pick up a free Sim card at immigration (one per person) and use the free 50rs credit to make a couple taxi bookings.