There's a new Steam Beta client available for download that adds a new remote management feature for the Valve-owned digital games distribution service. A forum update explain it pretty succinctly: using a web browser or mobile device, those with the beta client installed can remotely download owned games to their computer.
To try out the feature, you'll first need to install the beta client. Open Steam, go to the "Steam" pull-down window, and click Settings. In the Account tab, under Beta Participation, click Change to get yourself set up. Once the beta client is installed, just visit www.steampowered.com from any browser, visit your games list, and click the install button for the desired game to start a remote download. The only requirement is that the Steam client be running already on your home computer.
You can find the official remote management instructions right here.