I'd recommend finding yourself a used WRT54G (v2 ~ v4) or WRT54GL and flash it with DD-WRT firmware [dd-wrt.com]. I usually pick these up on eBay for around $20.00 or so and flash them with aftermarket firmware (either the aforementioned DD-WRT or OpenWRT [openwrt.org], although I find the former is much easier to setup as a wireless bridge). IIRC, I've even seen a couple of hacks where PoE was used with a WRT54G to put it on the roof.
Just be sure you get a model with a supported version. Some other routers (e.g. Buffalo) can also be flashed and work the same way, and can be found cheap.