LarryD:
These are excellent choices for high current applications:http://www.adafruit.com/datasheets/TIP120.pdf
http://www.redrok.com/PNP_Darlington_TIP127_-100V_-5A_Hfe1000_TO-220.pdf
Darlingtons like those can carry a reasonable amount of current and have enough gain, but that comes at the price of a high saturation voltage (typically 2V or more at an amp or two) and running hot. You are better off with an ordinary medium-current transistor (not a 2N3904 or other low-current transistor) for switching current up to about 0.5A, and a mosfet above that.