Steam announced that they will launch a new feature in a few days that will allow you to share your whole Steam Library with up to 10 people. That is freaking awesome!
There are some restrictions though, while I'm sharing my library with a friend, both of us can't play at the same time. I have first dibs though, and if my buddy is busy playing a game, and I start one up on my side, then he'll get a chance to buy the game to continue, or his game will stop. Sounds fair!
Your friends can then have their own save games and Steam Achievements.
I think this is a really awesome move from Steam, I expect nothing less of them, they always seem to surprise me!
It's starting the beta next week, and to participate, you need to join this group on Steam. And here is more info about the system on Steam's website.
Don't get your hopes up though, it seems that they'll only select 1000 people to participate at first (and the group already has over 100000 members!)
Anyway, nice one Steam!