I've been through all kinds of fans trying to get silence (first) and reasonable air flow (second) and my favorite by far is the Arctic Cooling F12 running at 5V. It starts up in the mid 3V range so running it at 5V isn't a problem. It is a cheap FDB fan (under <$10) and the current version (the F12) is even slightly quieter than the older versions. I used to use Zalman Fan-Mate to get the older versions to around 4.5V, but the F12s are quiet for me at 5V so now I have a bunch of Fan-Mates in a box and use Zalman ZM-MC1s to generate the 5V without any power loss through the Fan-Mate.
I actually use theses things all over the place. I have one sitting on my desk to cool me off (try it, just a little airflow on yourself constantly makes a big fan unnecessary). I have a couple hooked up to battery boxes in my emergency kit (I'm in Houston so staying cool in a power outage is a big problem). I have a couple aimed at laptops sitting on Laptop Lifts (http://www.gpg2.com/
). As strange as it sounds, I even have on my night stand aimed at my head when I sleep... and just about any noise bothers me when I sleep.
That one on my nightstand was actually hooked up to a $100 variable desktop power supply (that was the older Arctic Cooling 120mm fan before, now I run a F12 at 5V from a USB power charger). It displays the active current delivered to the load, which turned out to be something like 40mA at 5V which is amazingly low (its spec is for 150mA at full speed 12V). When I hook up ball bearing fans to that same PSU I usually see 3 to 4X the power consumed for the same quietness/airflow.
I use smaller (40mm/60mm) low RPM SilenX fans like the IXP-34-12 where 120mm fans won't fit and they are also quiet. But I agree the the other posters, they aren't as great as they are built up to be.