I personally hate loud systems, so I did a lot of looking around for quiet fans. I learned two things: if you can fit them, it's much better to go with 120mm fans that have a slow rotational speed, because they are generally quieter than 80mm fans that spin fast (while pushing as much or more air).
Secondly, make sure that you have sufficient openings in your case to get the air through. I bought a PC Power & Cooling (pcpowercooling.com) "Silencer Auxilliary Fan" (which I would recommend- as it is virtually silent!), and put it in my case. When I held the fan in my hand in open air, I couldn't hear it at all, even from up close. When I put it in my case though (front-side bottom), it still generated noise. The reason is because my the metallic inside in my case (not talking about the front bezel) had a bunch of little air holes where I put the fan. As the fan drew the air through these holes, it caused noise. I took a pair of tin-snips and cut a big circle the size of the fan where the air holes were. Now the air-flow is improved, and the noise has disappeared.
Bottom line: get a silent fan and modify your case to make sure the air-flow to the fan isn't limited or pulling through tight little holes. Also remember that larger fans with slower rotational speeds are quieter while moving as much air. And lastly, remember that "silent" fans usually have the trade off of not moving as much air.