In my experience the best quiet fans are at least $15 and made by brands like Noctua,Scythe,Nexus,and Thermalright.
Most use FDB.Fluid Dynamic Bearing.
You can expect the db ratings claimed by these makers to be accurate within one or two db.
Don't get caught up in cfm either.
Here's a quote taken from a very good site SPCR that actually has real testing facilities.
"Finally, CFM has no direct bearing on cooling, which is measured not by airflow but by a drop in temperature, usually in a device in the PC. Without the benefit of a thermal engineer's knowledge and detailed parameters about the components and conditions, CFM might as well be APH (angels per pin head). The relationship between CFM and cooling is at least as complex as that between SPL and perceived noise. The CFM value has no real meaning beyond itself. In contrast, with a bit of experience, 30 dBA/1m does have some meaning. Still, DIY computer tech geeks want to compare fans by their CFM rating, and in the SPCR (and other PC tech web) forums, some have gone so far as to specify what CFM rating they believe is needed for their application. This is a reliance on CFM numbers that has obfuscated the role of airflow in cooling. It's not really a surprise; CFM is one of the very few performance specs that fan manufacturers make available.
Over the years, we have observed one clear phenomenon about fans and cooling: The relationship between airflow and temperature invariably becomes exponential at some point. Increase airflow from nothing to something, and the drop in temperature can be dramatic. Keep increasing airflow, and the cooling improvement becomes less and less significant, until at some point, the temperature hardly drops at all. The trick, for the PC builder who seeks both good cooling and low noise, is to find the point where any decrease in airflow (or fan speed) effects a significant increase in temperature, while only a very large airflow increase effects a significant temperature drop. In other words, once you have enough airflow, additional airflow has very little cooling effect, so all you're doing is increasing noise. "Enough airflow" is not a constant, of course, it varies for each system of components."
Good tested fans.
http://www.silentpcreview.com/article63-page2.html
Nexus BASIC D12SL-12
http://www.newegg.com/Product/Product.aspx?Item=N82E16835610006
All the good Scythe's are out of stock at the Egg and Noctua's are ugly and overpriced imo.
http://www.frozencpu.com/products/2505/fan-205/Scythe_S-FLEX_SFF21E_120_x_120_x_25_mm_Fan_w_S-FDB_Bearing.html?tl=c15s60b14
http://www.frozencpu.com/products/10218/fan-662/Scythe_Kaze_Maru_2_Slip_Stream_140mm_x_25mm_Fan_120mm_Mount_-_1200_RPM_SM1425SL12M.html?tl=c15s775b14
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