This is another cheap alternative to using Microsoft's xbox 360 wireless gaming network adaptor and should hopefully allow you to connect your xbox 360 to xbox live wirelessly.
Essentially this technique involves converting a router into a wireless receiver! How is this magic achieved you ask? FLASHING the router with new modified firmware. The administration software that is normally present on your router will be erased and replaced with a modified version.
DD-WRT is a third party developer of modified firmware that will transform your router into a receiver. Not all router's can accept this firmware so check this list first: Supported Devices
If your router is not compatible then try to pick up a linksys WRT54G router. They should be cheap (as they are old tech) and they work really really stinking well with DD-WRT firmware. You will likely have very little problems with this model of router allowing you to easily connect your xbox 360 to xbox live wirelessly.
For installation instructions it is best you refer to this site as there are SO many frigging different types of routers supported it would be difficult to cover them all here:
To set up the router you need to access your modified router by connecting it to a pc via an ethernet cable. Use a browser and type 192.168.1.1 and you will then be prompted for a username and password. The user name will be root and the password will be admin.
You then have access to the modified administration software and you need to change the router to wireless client mode.
So hopefully you will now be able to connect your xbox 360 to your modified router via ethernet cable and that router in turn communicates wirelessly with your primary router that is connected to the internet. You should now get xbox live on your xbox 360 wirelessly.
You will most likely have issues along the way and DD-WRT wiki is the best resource to iron out those tricky problems. Good luck! Modified routers usually give better reception then conventional xbox adaptors allowing your xbox 360 to connect to xbox live wirelessly with blazing speed :)
Now go connect wirelessly to xbox live using a modified router !