This is a daemon that uses input from coretemp module and sets the fan speed using the applesmc module. It includes a PKGBUILD for easy installation in Arch Linux.

Based on:

http://allanmcrae.com/2010/05/simple-macbook-pro-fan-daemon/
http://allanmcrae.com/2011/08/mbp-fan-daemon-update/
https://launchpad.net/macfanctld
http://paste2.org/p/862259
http://www.lobotomo.com/products/FanControl/