Single board computers
From Wildsong
RouterBoard
I currently am experimenting with a RouterBoard 532A (about $200 including a case and power supply).
Manufacturers
PC Engines makes WRAP boards, $129
Soekris boards are also popular for WiFi projects.
RouterBoard 133 model is just $59
PepLink makes load balancers, WAPS, and a general purposes gateway router/switch box calld the Manga. See also "Debian in 3 watts". This shows how to set up a Manga to run Debian from a USB storage device.
Metrix Wireless development kits