Comment iMac Desktop (Score 1) 371
I am using an 20" iMac Desktop from 2008 or so. I have it set up to dualboot into Win7 using Bootcamp, and I admit I primarily spend my time on Windows. It is more or less completely silent, I can hear a slight sound from the fans but its almost always drowned out by sound from other sources (traffic, fridge, cat snoring etc). To the point that I was surprised to read this poll because I hadn't thought about system noise in ages.
Whether or not you like OS/X, the iMac is a brilliant desktop computer. Mostly I spend my time actually doing something with it rather than thinking about the system, unlike a lot of my Windows PC desktops prior to 2008.