Independent fans from the PSU and Heat Sink ones. You should always have 2-3 of these in your system.
These fans are usually 80mm or 120mm in side length.
Installation
The power cable for the fan(or heatsink if it has a fan) is a 4-wire standout on the motherboard.
This allows you to configure your fan in the OS, but you rarely have to as most fans are self-maintaining.