Just in time for Christmas, Steam is rolling out a new gifting feature on their gaming downloading service. To send a gift you just need to sign in to your account, select the game and then select it as a gift and indicate who it's for. The recipient will receive an email saying they've gotten the game and, if they're new to Steam, they'll get instructions on how to sign up.
Hit up the official Q& A for all of the lovely gift-giving details.
Steam Gifting [Steam]