TRENDnet has one too:
http://www.trendnet.com/products/proddetail.asp?prod=185_TEW-647GA&cat=66
It works much better than the integrated one in the Wii, especially if you have a Wireless N router or access point... The only problem is that you have to use the USB to ethernet and then the ethernet to wireless N. I suppose that in the case of the XBox360, you can plug it directly to the ethernet port.







