As TR's resident fan-control nerd, I've spent far too much time critiquing how motherboards go about adjusting fan speeds in response to system temperatures. Some boards offer plenty of fan control options in their firmware, while others use software found on the accompanying driver CD. Sometimes, and especially with older boards, neither of those options provides a sufficient level of fan control. What's a noise-obsessed enthusiast to do?
Install SpeedFan. The free application has all kinds of neat monitoring and fan control functionality, although it can be a little cumbersome to configure. Fortunately, Silent PC Review has put together a SpeedFan setup guide to help get you started. The guide walks through the configuration process and details the various options available for intelligent fan speed control.
Most new motherboards do a passable job of regulating fan speeds, but few offer the level of control provided by SpeedFan. A recent version added advanced fan options, including adjustable response curves with user-defined hysteresis. Motherboard makers would do well to take notes, and it would be nice if they worked closely with SpeedFan's developer to ensure the app correctly detects temperature sensors and fan controllers on new models.