Recently, there was a rumor going around that Valve were working on creating some sort of game share feature for Steam. As it turns out, this was true as today Steam Family Sharing was revealed. This is an upcoming feature for Steam that will allow users to share their extensive game libraries with family or friends.
Basically, the lender authorizes a friend or family member to have access to their account. A lender can authorize up to 10 other people. Those users will have access to the entire library - at least as long as no one else is currently playing. Only one user can access the games of a shared library at any time. But if the lender themselves wants to play a game, then the borrower will always have to be kicked out after a few minutes.
What other sorts of restrictions are there? For one, you cannot share a game with someone if it requires some sort of activation key or secondary account. This makes sense, at least. The same holds true if you are trying to share across regions and one game is not available in both.
If you're interested in getting into the Steam Family Sharing beta then join the official group. Later in September, 1000 users will be randomly invited to the beta. As of right now, there are more like 100,000 members in the group. Hopefully we will see a larger roll out of the feature shortly after the first beta group gets in.