Heat Sink Aluminium Extruded
Here is a lovely little aluminium heatsink with fins for a typical voltage regulator such as a 7805 which is typically available in a TO-220 package. This heatsink can also be used for a transistor in a T0-220 package as well. It has one hole to fix the voltage regulator to and as you can see it stands upright.
This is called a passive heatsink because it's heat is simply lost into the surrounding air - which is how it remains cool. There are also active heatsinks which have a fan fitted to them however they are mainly for CPU's.
Ideally you need the heatsink to be as large as possible - that the PCB can allow. Upright standing ones such as these occupy much less floor-space on the PCB making it ideal for those compact projects. Heatsinks are typically made of Aluminium because it is a good conductor of heat and relatively inexpensive. The fact that aluminium is also light is a huge advantage for weight-critical applications in avionics.
There are two pins that can also be soldered to secure it to the PCB. If you've got tracks on the component side you might also want to use a few layers of mica to insulate the aluminium from the tracks.
When fixing the component to the heatsink be sure you apply some thermal paste compound to ensure good conductivity between the two surfaces. Thermal paste is also available from my store.