As noted on Steam News Half-Life 2 is now officially available at retail stores throughout North America. Though Valve's highly anticipated sequel has been available early at some over-caffeinated stores (story), the online activation process is now underway, and copies of the game purchased at stores or online via Steam can now be unlocked by restarting Steam:

Half-Life 2 is available now for purchase and to play. Those who pre-purchased their copy via Steam may access the game by restarting Steam, then double-clicking on the Half-Life 2 icon in their Steam Games directory. To purchase your copy via Steam, get Steam now.

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