Nexus, of NX-3000 PSU fame, has recently introduced a series of new quiet PC components over the summer:
The Nexus 80mm Real Silent case fan is said to produce only 17.6 dB(A), this value "the result of numerous tests in an anechoic chamber following the highest standards." They say they "simply cannot find another [fan] this quiet!
The NX-3500 Special Edition, a 350W power supply featuring the increasingly popular 120mm bottom-mounted fan. They claim it is only "19.2 dB(A) in idle mode and will be inaudible when mounted in a computer system as the fan will be located on the inside of the computer case. The NX-3500 has longer power cables and is SATA ready!"
Then there is the all-aluminum PHT-3600 SkiveTek, a light weight P4 heatsink equipped with a fan with 2400 RPM and a noise spec of only 19 dB(A). It is said to challenge all-copper HSF in performance despite its low noise and aluminum construction.