Valve's Steam platform is going to be getting a nice boost soon, as the company announced Steam Family Sharing. It's a new service that allows for Steam users to share their entire libraries with family and close friends on up to ten different devices. Each shared computer gets their own set of acheivements (so your buddy/brother/sister/dad/mom/other relative can have a fresh experience), with all saves and application data being stored in the cloud. However, the one caveat is there's no simultaneous use when Family Sharing is running. So that means if you, the lender, start playing a game the same time as a shared user is – even if it's a different game – the shared user gets a few minutes to either purchase the game they were playing or quit out.
A limited beta for Steam Family Sharing begins next week, and the best way to secure access is to join the appropriate Steam group. In order to share your Steam library, you have to either enable it in your account settings or grant access to requests sent by other users. Regional restrictions still apply, and lenders can even have sharing access revoked if a borrower cheats in a game. Pretty much every game can be accessed by the shared users, however if it requires "an additional third-party key, account, or subscription" then that game can't be shared. DLC can also be accessed, however only if the lender actually owns the DLC; a shared user can't buy DLC for something they personally do not own.