You may, this very day, start downloading a Steam game on your computer at home by loading up your web browser at work, to have it ready to go when you arrive back home. Valve introduced remote downloads in a beta build earlier this month, but the feature was officially patched into the regular Steam client yesterday.
To download a game remotely, simply log into the Steam Community site in your browser and poke around your games library. The PC you're downloading to needs to be turned on and online, obviously, and you need to be logged into Steam Friends too. One imagines this will be added to the Steam mobile app soon too.