If you've got burnt thighs from an old MacBook Pro or you notice that your computer is always overheating and crashing from using CPU-intensive apps (like playing PC games in a virtual Windows environment), you might want to check out smcFanControl. This free, GPL-licensed utility has a single purpose: letting you increase the minimum speed of built-in fans, so your Intel computer will run cooler.
smcFanControl lets you monitor the current temperature (in Celsius or Fahrenheit), assign different minimum speeds for each fan using sliders, and even apply different settings when your power source changes (for example, going back to default fan speed when using battery power). As with any settings-tinkering software, you'll want to use smcFanControl judiciously--but fortunately this application always keeps fans in automatic mode (so speed will increase along with CPU load) and never lets you set the fan speed below the Apple-recommended minimum. This most recent version adds support for every current MacBook, MacBook Pro, MacBook Air, and Mini.