I put a small fan in mine and found it cooled things really well but was too noisy and not having the electronics will to put in a thermostat or resistor to turn it down a bit, took it out.

For anyone interested, there's a pretty nice and detailed thermistor-based fan project available in kit form, linked from the FAQ.

In fact, I'm considering doing that very project myself after buying a really quiet and low profile 40mm fan. The fans in my docking stations work, but I've got no cooling in the car and sometimes it gets pretty hot. Is there a consensus as to what the quietest low profile 40mm fan is?