ATI XT Vouchers for Half-Life 2
These are available with the purchase of a 9600 XT or 9800 XT

In the past, you had to create a new account or such to enter your HL2 CD-Key into Steam immediately. You could alternatively wait for HL2 to show up in the My Games list, however that would not allow you to play the Counter-Strike: Source beta release(s).

Taylor Sherman has told us that in the upcoming Steam update, their will be a new method to enter the CD-Key and still maintain your existing account, without having to wait until HL2 is released. This is good for those anxious to get the jump on for the CS: Source beta(s).

UPDATE: The update has been released and owners of the Vouchers with CD-Keys will now be able to enter those keys into the Steam platform in order to gain access to the Counter-Strike: Source beta.

To enter a CD-Key you have, go to the My Games window and click Browse games... at the bottom. From there, a window will pop-up and you should click at the bottom Register a product I already own... and enter your CD-Key.