Valve have released a Steam beta client which has a new feature tucked into it called Remote Downloads. Remote Downloads is a feature that lets users manage their game libraries remotely. This means that users can login to the official Steam site and have access to their game list.
From here, they can start downloads and monitor their progress. The only requirement is that Steam has to be left running on their PC, which is obvious. Because Remote Downloads is done through a standard web browser, this means that smart devices like smartphones and tablets should be able to get in on the fun.
At the moment, the Steam apps for iOs and Android allow game purchases, but don't play well with a home installation of Steam. This function should be baked into the app once Remote Downloads makes its way out of beta, and into Steam as a proper feature. Nice work, Valve!