The Fortron Blue Storm PSU's are nice and use a single 120mm fan which is nearly silent. Thats the only one Ive had experience with that was made for quiet operation.
hiper psu seconded and its quiet but i noticed tho when playing militia on css the fan speeded up for the first time and it got quite noisy but nothing annoying really
I've had good results with Fortron made psus, and they came out top in a reliability survey I saw recently. I've had FSP, Nexus, and Aopen versions, all much the same, nice and quiet, just pick the cheapest.
Fortron's also come highly regarded as very powerful and stable overclocking PSU's. I loved my Blue Storm, I just grew out of the 400w range quickly when my watercooling went in