Comment RouterStation PRO by ubiquity (Score 1) 334
It ships with (an old version of) OpenWRT preinstalled. It doesn't get better/friendlier than that
Add a nice case (for instance from netgate.com - they have them but the page for that product seems to be broken right now, sigh) and powersupply (48V DC, netgate has them too).
Finally, add up to three minipci wifi cards (and make sure to get pigtails and antennas). A good vendor for that stuff is pcengines.ch; the Wistron DNMA92 Atheros 802.11a/b/g/n card is cheap at $26 and it uses the ath9k driver (no binary blobs). PCengines also has cheap pigtails and antennas.
All in all this costs quite a bit more than your run of the mill access point, but this puppy is a lot more powerful than your average access point.