For example, the motors described on this site may experience peak current over 1A at 6V. Usually this happens when the rotor is stalled and the transistor is open. You should not leave the motor in this state as it may destroy the transistor. When the rotor spins with a proper speed the transistor turns on and off, and the average current flowing through the transistor is about 10 times smaller. If the collector current through the transistor is high, the transistor may get extremely hot and burn the fingers if you touch it. Normally, the motors described on this site do not need it, but you may still add a heat sink if necessary.
Some motors require the transistors that are more sensitive.
For that purpose certain kits use so called Darlington transistors. Darlington transistors are actually two transistors in one case where the second and more powerful transistor amplifies the signal from the first transistor. Darlington transistors look the same as regular transistors. The designs on this site represent the most simple circuits. However, all of them were tested extensively and proved to be very reliable and worked very well.