Linksys WGA54G Wireless-G Gaming Adapter

While this is not as sleek as the Xbox 360 wireless adapter that snaps on the back of your Xbox 360, this is a cheaper wireless alternative.  In addition, it will work for old PC's without a wireless card (via ethernet plug) or other gaming consoles.  I have tested this with the Xbox 360 and it worked great. 

We checked around for the best price (new) and Amazon has it for $69.99 (save $10).  So, you can save around $30 over the official Microsoft wireless adapter.

Check it out: Amazon - Linksys Wireless Gaming Adapter 

Product Description
The LINKSYS WGA54G Wireless-G Game Adapter gives any wired Ethernet-equipped game console 802.11g wireless connection capabilities. If you have an existing home wireless network and a cable or DSL Internet connection, the Game Adapter connects your PlayStation2, Xbox or GameCube to the network (and Xbox 360). That way, you can get into online games without running wires to the game room. Or, use a pair of Game Adapters to form a "cable-less cable" between two game consoles, for head-to-head gaming -- in the same room, or all the way across the house.