While it's also not free, you can configure a wireless router/access point that supports either wireless bridging, WDS, or both to supply wireless networking to the 360. This solution will be a lot bigger than the 360 wireless adapter, but it's also a lot cheaper and you can use it for other things.
I use one of my two Linksys WRT54GL routers with the Tomato firmware in WDS mode to act as a sort of wireless switch to get my 360 on the network.