There are 2 ways off getting on to the Call of Duty 4 Beta for free.
1, Go on this website next friday (7th/9/07) and sign up to get your COD4 beta token but there limited so you need to be fast.
2, Pre-order Call of Duty 4 off http://www.Game.co.uk
and they will send you a code for the beta. And as soon as you get the code if you want you can stop your pre-order. Game only send codes out to your e-mail once a week.
Please note you will need to have a xbox 360 console and access to xbox live and xbox live gold membership.