I have a home-build temperature sensitive quadruple fan controller:
Basic design here:
(It's also in the LM317 datasheet along with other fun stuff you can build with it.)
It will not turn on the fans or so, but it will vary the voltage depending on temperature. You don't need the capacitors, they probably do something good in a sensitive environment, but the 12V lines in a computer nor the fans are that sensitive.
The LM317 can handle 1A (=6W at 6V, or 11W at 11V), IIRC. If you use bigger fans, you will need to work with power transistors and stuff.