There’s good news for all of you who’ve ever wanted to purchase a game from Steam while away from your PC and have it downloaded and installed by the time you get home. The latest Steam beta client finally puts this functionality at your fingertips!
The beta client is accessible via the Settings menu on your account page and allows you to purchase and manage Steam content while at work or on the go. You can also remotely download and install games, provided the Steam client is running on your target PC or Mac.
Source: Rock Paper Shotgun