SpeedFan is pretty damned neat. Open-source (free) program that monitors fan speeds, temperatures and voltages, and even show hard disk temperatures too, if supported. At the lowest level, SpeedFan is an hardware monitoring software, but its main feature is that it can control the speed of the fans (depending on your sensor chip) according to the temperatures inside your pc, thus reducing noise and power consumption.
SpeedFan can be minimized to the tray and is compatible with Motherboard Monitor 5.