I don't know whether a nice GUI app exists. Would be nice.
The grunt way to control fan speeds:
* Ensure lm-sensors is installed (true on a stock Slackware).
* As root run pwmconfig, which creates an /etc/fancontrol config file.
* Create an rc.fancontrol script or add something to rc.local to run /usr/sbin/fancontrol &.
* Add monitoring options to a conkyrc configuration or every 15 seconds run 'sensors' from a terminal.