Steam has been running a restricted beta since last last November that allowed users to share their library of steam games with family members or friends. Steam Family share has now gone live and is available to all Steam users worldwide.
The service allows you to share your library with up to 5 other account holder and can be authorised for 10 devices (including your own). It is impossible to play games simultaneously and the owner of the game always gets priority to play. Achievements are earned for the player rather than the owner of the game and save games are stored per user in the cloud. There are some other restrictions on individual games if they are region locked or need an authenticator or subscription but most games seem to be available.
Steam Family Sharing is relatively easy to set up – but there is one big caviat. You need to sign in on the remote machine. In other words if you are in Glasgow you want to let your friend in Southampton use your account you will have to physically go there and sign in to their machine for the first time. If you are willing to put in a little time and effort however, a program such as Teamviewer and a little coordination would let you and your friend access each others libraries.
With a little thought and possibly sharing with friends who play at different times or in different time zones Steam sharing represents a way to expand your library of PC games with no extra cost.