All you need for this is an xbox 360, an ethernet cable, and a wireless internetlaptop/desktop.

There are different ways for all OS's so iam going to give links to all three, and my own revised version for vista premium. (because i haven't actually tried this on a mac, and have only seen it work on an xp pc)

Windows Xp- http://www.noheat.com/2007/07/19/get-wireless-xbox-live-for-your-xbox-360-without-adapter/

1. Go to Start and go to "Connect To", if you don't have a "Connect to" folder in your start menu you can go to your Control Panel and click on "Network Connections".

2. Right click on your Wireless Network Connection.

3. Once in the properties of your Wireless Network Connection go to the advanced tab. If you are inexpereienced at sharing a network connection out you can click on the Network Setup Wizard and follow the prompts for sharing out your connection.

4. Once you have setup your wireless laptop to share out the wireless connection through your regular network connection simply connect the crossover cable to the laptop and to the back to your Xbox 360 and sign up for live. Be sure to have setup a hotmail account if you don't already have one, it is required for accessing live. Also beware that if your connection to your wireless router is spotty in that section of the house you will have issues connecting.

If this didn't work, and you are positve that you did everything right and meet all the requirments, thenPm me and i'll try see whats wrong.

Mac OS X- http://www.joystiq.com/2006/07/17/how-to-share-your-macs-internet-connection-with-your-xbox/

On your Mac

1. Connect your Mac to your wireless internet connection
   
2. Connect your Xbox 360 to your Mac via Ethernet
3. In System Preferences -> Sharing, enable Internet Sharing between your Airport and Ethernet connection

On your Xbox 360

1. Start up your Xbox and navigate to the Network settings menu
2. Under IP address, enter the number you got from the "ifconfig en0" command earlier on, but with the final number one place higher. In my case, I take the 192.168.2.1, and input 192.168.2.2 in the IP address box on my Xbox 360.
3. Under subnet, input 255.255.255.0
4. Under router/gateway, enter the number you got earlier from the "ifconfig en0" command. In my case, that's "192.168.2.1"
5. Move onto DNS. Enter the IP address of your wireless router (in my case, "192.168.0.1") in both the primary and secondary IP address fields
6. Leave PPPoE and Advanced settings blank (i.e. OFF and Automatic)
7. You should now be able to connect to Xbox Live via your internet sharing Mac

Like many people have said the "on your xbox 360 part" may not be necesasary, but i put it in there just in case.

If this didn't work, and you are positve that you did everything right and meet all the requirments, thenPm me and i'll try see whats wrong.

Windows Vista Home Premium (and all the others i think)-

http://blogs.gotdotnet.com/dawate/archive/2007/07/18/xbox-360-networking-with-a-laptop-s-wireless-connection.aspx

I tried this out and got a basic idea of how to do it, but (at least) for vista premium, it didn't work word for word. I did get it working and here's how.

1. connect ethernet cable to your xbox 360 and to your pc
2. leftclick on the two computer screens in the bottom right corner of your screen
3. then left click on either of the connections (either your home network,or the unidentified network.
4. that brings your to network sharing, right click on the xbox 360, and click bridge connections
5. then on your 360 go to join live, test connection (yes), wait about a minute, and then browse xbox live!!

If this didn't work, and you are positve that you did everything right and meet all the requirments, thenPm me and i'll try see whats wrong.

I take almost no credit for this how to guide, and all of the credit goes to the authors of their respective web-page links, because without those links i wouldn't have known how to do anything. Thank you all for reading and enjoy Xbox live.

P.S. my gamertag is AnAssassin7