DD-WRT, the awesome open source router firmware, has a feature that will turn your router into a wireless client instead of an access point. In this configuration, a cheap 802.11 access point can be transformed into a wireless adapter that you can plug standard ethernet-bound devices into.
This project will help you change a cheap wireless router into a wireless receiver for your Xbox 360. The total cost of this project can be as low as twenty or thirty dollars, compared to the one hundred dollars Microsoft wants for their little wireless dongle.
If you have a PC or two without a wireless connection, this hack is also a nice way to extend your network throughout your house without needing to pull any cables. I have a feeling that the hardware in most routers will pick up a stronger signal than built-in PC/laptop wireless adapters, so it may also be worth checking out if you get a spotty signal in your corner office.
DD-WRT Client Mode: Cheap Wireless For Your Xbox 360 – Link