One dollar variable fan regulator
Searching for an easy to use fan regulator for some small 12V fans, I came across a 7805 app note from TI. It describes how to use that chip as a variable regulator, giving ca 5-10.5V out (when fed with 12V). Perfect for regulating a 12V fan.
The app note specifies a 330 nF input capacitor, I went for a 100 nF as that is what I had. Works great.
Only downside is that a linear regulator such as the 7805 is not very efficient => lots of heat will be result when setting output voltage to 5V.
Turns out several commercial fan regulators for PCs are built exactly this way too, including the Zalman Fan Mate II.