I've been playing around with the Linksys WRT54G and GS models. These sport a 200Mhs MIPs cpu and you can flash the firmware to run custom Linux builds.
Check out http://docs.sveasoft.com/ and http://openwrt.ksilebo.net/ Linux 2.4.20 kernel, iptables, QoS iproute2+tc stuff all built in. Horror stories - you can flash the firmware and screw yourself so be careful. These Linksys routers range from $60 - $120 mailorder - retail. A really cheap router. No fans too, and only 12V.
