From the airport to Cambridge, it's metered, but from Cambridge back to the airport, it's a flat rate.  In 2006, it was $38 - INCLUDING tip and tolls.  Check the back of the driver's seat as the rate should be prominently posted.

Taxis are pretty easy to find in the major squares within Cambridge or at hotels, but you will most likely need to call one if you are in a more residential area, as they won't be easy to flag down.

Local cab companies include:

Cambridge Checker Cab
103 Fulkerson St,
Cambridge, MA
(617) 497-1500  

Cambridge Union Taxi
2464 Massachusetts Ave # 440,
Cambridge, MA
(617) 661-2500  

Ambassador Brattle Taxi
76 Hampshire St,
Cambridge, MA
(617) 492-5010  

Cambridge Cab Co
48 Fresh Pond pl,
Cambridge, MA
(617) 354-5005  

Yellow Taxi
580 Cambridge St,
Cambridge, MA
(617) 494-1500  

Twin City Taxi
207 Magazine St,
Cambridge, MA
(617) 876-3332