smcFanControl

smcFanControl is a little free app that allows you to control the fan speeds (and hence the temperature) of your Mac. It works on most Macs but not all. I use it a lot to crank up the fans when I have a long encoding/transcoding or rendering session that I leave overnight.

It runs as a little menubar item which shows the CPU temp and the rpm of your fan(s). You can set preset fan speeds like one for Rendering and another higher speed for Encoding and another for Gaming if you want. On top of that it, running it seems to have fixed a reluctant-to-speed-up fan in my GF's Mac Mini which was beeping as the fan wasn't spinning up in response to increased CPU usage and therefore heat so the overheating beep or tone was sounding every few minutes. (About 185°F @ 2700rpm seems good for an Intel 2Ghz Mac Mini at ≈ 98% CPU usage.)

smcFanControl is even safe to run constantly and set to run at login because all it does is modify a few plists and read the temperature sensors built into your Mac. What it does is set a minimum fan speed, allowing the fan to ramp up as needed as temperature increases, also, it won't allow you to slow down the fans to anything below the default speed Apple has set so it's quite safe to use. I've been using it since release 1.0 without any problems. Plus it's just fun to watch the temp and fan stats update in the menubar.

It's simple, free and could end up prolonging the life of your hardware. Plus their website has girls on it.