Hey dg, this is a great question, one that I've been interested in for a while. I'm definitely curious to see what other people recommend. I've found it's very difficult to find a fan that's both powerful and quiet. The best I've found are Panasonic's whisper line of bathroom fans. I don't have one yet, so I don't know how they actually perform, but their specs look good. Here's one that's rated at 80 cubic feet per minute (good enough for a cabinet grow I think), with a sound rating of 0.3 Sones (don't ask me what the hell a sone is. For some reason that's the unit of sound measure for bathroom fans rather than dB). Panasonic FV-08VQ3 - Panasonic Whisper Quiet Fans - WAMHomeCenter.com

Anyway, I'll definitely be interested to see what other people recommend.