Earlier this month, Valve Software announced their new service dubbed Steam Family Sharing. There's no need to get your gray cells started, for Steam Family Sharing allows you to do exactly as the name suggests, i.e. share your pre-owned games with your family members (not just your brothers and sisters in the real world, but your entire friends list from Steam, which is being referred to as a family here).
Starting today, the lucky 1000 who have been sent invites to participate in the beta testing phase of Steam Family Sharing can add up to 10 users with whom they can share a game. Don't you worry, all your saved games and achievements are separate from the other sharers, but only one of you can play the game at a time.
Check out the FAQ for a full list of Do's and Don'ts.