Windows only: Free application SpeedFan monitors the fan speed, temperature, voltage, and more of the components inside your computer. That's all fine and dandy, but the marquee feature of SpeedFan is that you can actually adjust the speed of your fans by tweaking the desired temperatures on supported systems. Keep in mind that SpeedFan is a power-users tool, and the temp inside your computer isn't to be taken lightly. However, if you're comfortable tweaking hardware settings, an app like SpeedFan can help keep your computer running quieter and give you a little more control over what's happening with your computer's innards. If you've used SpeedFan in the past, share your experience in the comments. Mac users, check out previously mentioned smcFanControl.