I have used Noctua fans and Pabst fans in the Pabst (much harder to find) for quieter, but cooler fans in builds. These are both excellent for what you're after. Here's what I recommend:
NOCTUA:
NF-F12 PWM (higher airflow, slightly noisier, but still near silent, more expensive)
NF-P12 (slightly lesser airflow, quieter, but still near silent, less expensive)
The Noctua NF-F12 PWM, for instance, has better stats than the Arctic mentioned above (higher airflow, same noise rating), but they're also more expensive. The other drawback to the Noctuas for me is that they're a bit on the ugly side, but if no one's going to be seeing them through a windowed case, who cares.
Also, keep in mind the pin connections you need (E.g. 3-pin vs 4-pin). A relative newcomer is Be Quiet! I have a few of their fans, and they're definitely quiet, but the airflow isn't as good with the Be Quiet! Silent Wings I have than the Noctuas.