Valve just released a press release stating that an all new feature, called "Family Sharing" is coming to Steam (in limited Beta) on Steam next week.
Family Sharing, which will allow for close friends and family members to play each other's Steam games , as well as earning Steam Achievements, storing their own saves and app data to the Steam cloud. All it takes is authorizing a friend or family member's computer for use with your Steam Library!
Once you authorize a computer it opens up your library to the person that you shared it with, allowing them to download and play. One thing to note, simultaneous gameplay is not allowed, but the person doing the lending can play their games at any time. So if you hop into a game while a friend or family member is playing, it will send them a message allowing them to either quit playing or pay for the game.
This feature only augments the vast number of features that we have come to enjoy from Steam. For more information about the Steam Family Sharing Feature and the all new beta program, check out this link!