Transistors

Use transistors as a switch to control power to motors, relays and lamps. Pay attention to peak current which transistor can handle. Also pay attention to Vce-sat, the voltage across the transistor when it is switched on. Vce-sat times the collector current is the power which the transistor must dissipate. Too much and the device will first smoke then melt. Current and power handling capability is pretty much dictated by package size. The bigger the package, the more power.

TIP120
NPN power darlington, TO-220 case, 60V, Ic max about 1.5A, beta about 1000 @ Ic = 3A.
Data sheet (PDF file)
Radio Shack 276-2068 ($1.29), Jameco 32903 ($0.65)
Pinout: hold by legs with legs down and metal tab in back. From left to right the legs are base, collector, emitter. The tab is also the collector.
Vce sat data when driving base from 5V through a 10K resistor:
0.7V @ Ic = 60mA
0.9V @ Ic = 210mA
1.0V @ Ic = 1.0mA
1.3V @ Ic = 1.7mA
Comments: Good for medium motors and lamps. Because Vce sat is significant, will quickly heat up if controlling more than about 1.5A. Can turn on by driving base from a Stamp pin through a 10K resistor.

2N3904
NPN general purpose, TO-92 case, 40V, Ic max 100 mA, beta about 100 @ Ic = 10mA.
Radio Shack 276-2016 ($0.59), Jameco 38359 ($0.12)
Pinout: hold by legs with legs down and flat side of case facing you. From left to right the legs are emitter, base, collector.

Vce sat data when driving base from 5V through a 10K resistor:
0.2V @ Ic = 60mA
Comments: Great for driving small motors and small relays, anything which require 100mA or less. Cheaper and lower Vce sat than the TIP120. Can turn on by driving base from a Stamp pin through a 10K resistor.