We've enjoyed the recent explosion of semi-passive graphics card coolers that stop their fans while the GPU is idle. Arctic Cooling is extending that idea to the CPU cooler with its Freezer 32 series of heatsinks. The company is a little cagey about how its semi-passive system works, but it appears that a controller built into the Freezer 32's 120-mm fan can shut off the spinner entirely when the PWM signal from the motherboard falls below a certain duty cycle threshold. Arctic says most boards can't deliver PWM duty cycle signals below 40%, so we imagine that's the magic number. As a result, no external hardware or cabling is needed: the fan plugs right into the CPU fan header, just like it would with any other heatsink.
Beyond the clever fan, the Freezer 32 uses a 5.9" (150 mm) tower with four direct-contact heatpipes running through its fins. The Freezer i32 works with Intel's most recent sockets from LGA1156 on, while the Freezer a32 looks ready for any AMD CPU or APU using Socket AM2 or newer. Arctic Cooling also offers a "continuous operation" version of the Freezer 32 that's supposed to be used in servers or other systems that are on 24/7. This cooler's fan is built with a beefier bearing than its stablemates'. The Freezer i32 and a32 carry $50 suggested prices, while the continuous-operation version costs $10 more.