One of the biggest features the Xbox 360 is missing it an internal Wifi adapter.  It's hard to believe they haven't made this option standard considering the PS3, Wii and DS all have it.  Granted I'm not too sure how high quality the device is on those other game systems but still it seems like an item that should have been included this generation. 

What Microsoft does offer is a $100 USB Wifi connector whish does work very nice and can be foudn for cheaper (like I did).    Still Gizmodo has an article (three years after the cosnole has launched) on how to get around the price barrier to Wifi on the Xbox 360.  Probably the reasonable and most Duh! one is you can use the original Xbox's USB wifi addapter.  That is exactly what I did for a good two years.  You can probablt find one for under $50 as some stores.  Gizmodo has a few more evasive options so checkout their article if you're interested.

Source:
Gizmodo