Either
a) Linksys WRT54G or GS (pre v5 devices, which have been crippled and no longer "easily" support Linux based firmwares) with one of the alternate firmwares
b) Buffalotech WHR-HP-G54 or WHP-G54S with one of the alternate firmwares
c) There are many other models which support alternate f/ware, but Linksys and Buffalo are the most wellknown ones
My personal favourtie is DD-WRT (
www.dd-wrt.com). Has QoS, very SIP friendly, plus a HOST of extra features (dyndns included).