Steam is adding support for game sharing. The in-development Family Sharing feature is meant to allow "close friends and family members" to share their entire Steam libraries. Gamers will be able to grant access to as many as 10 different devices, although only one will be able to access the library at any given time. People playing shared games will be able to accumulate their own achievements and store saved games in the Steam Cloud. They won't be able to buy DLC add-ons for games they don't own, though.
Valve cautions that some games won't support sharing. "Titles that require an additional third-party key, account, or subscription in order to play cannot be shared between accounts," the firm says. Translation: if the DRM isn't Valve's, sharing probably won't work.
What if you want to get in a few Counter-Strike rounds while your buddy is mooching your copy of Left 4 Dead 2? According to Valve, the library owner will always have priority. Your friend will be given a few minutes to finish up before they're booted. They'll also be given the option to buy the game outright—a clever ploy that could boost sales if borrowers can't help but want to play just one more level.
Sadly, the sharing system is restricted to entire libraries. The likelihood of conflicts would be much lower if games could be shared individually.
Family Sharing is rolling out first in a limited-access beta scheduled to begin in mid-September. If you're interested, you can throw your name into the hat by joining the Steam Family Group. Initially, a thousand accounts will be selected for the beta. It's unclear whether sharing will be allowed only within that group or if any Steam user will get to participate as a borrower.
If you manage to get in on the beta, be careful who you share with. Sharing rights may be revoked "if your library is used by borrowers to conduct cheating or fraud." That warning applies to Family Sharing in general, not just to the beta.