The boys at I am Not a Geek check out the Thermaltake Fan Speed RPM Switch.
It's a pretty good idea, check out what they thought here.
In their ever-expanding line-up of thermal solution products is the Thermaltake
Fan Speed H.M.L RPM Setting Cable, P/N: A1268. Although not Thermaltake's most
advanced offering, its simplicity may be its strongest selling point, and could
provide a quick fix for people looking to control a fan's speed without buying/building
a full-blown fan bus.