Valve is to let Steam users share the video games with friends and family members.
The Family Sharing initiative will let a user of the Steam service share their library of games across up to 10 other devices.
Other people will be able to play the titles, store their own saved games and earn their own in-game achievements.
Valve said a small scale trial of the sharing system would start in mid-September.
Steam is one of the most popular platforms used by PC gamers to buy and access video games bought over the internet.
Valve uses it to promote its own titles as well as games written by third-party developers, from whom it takes a cut of the sales.
By introducing the sharing facility Valve may make its service appear more attractive than that of rivals such as Gog, Desura and Electronic Art's Origin.
However, it could also impact sales if people who would otherwise have bought their own copy now decide to share someone else's.
To address this risk Valve will prevent more than one person being able to play the same purchase of a title at the same time.
Industry watchers say that if the move proves popular other online marketplaces may follow.
In an explanation, posted to the Steam website, Valve said free sharing would be enabled when someone authorises a computer to access their games.
Other Steam members can request authorisation if they see that a friend or other family member owns a game they want to play.
If the owner of a game wants to play it while someone else is already using it, the person borrowing that game will be given a few minutes to either buy the game for themselves or have to quit.
Sharing will give people access to almost any game stored in a Steam account not just those titles made by Valve.
However, Valve said not all games would be eligible for sharing because "technical limitations" - which might include separate subscriptions - might stop some being made available.
Valve spokeswoman Anna Sweet said the sharing service was launched in a direct response to user requests.
Valve is well known for video games such as Half Life, Left 4 Dead and Portal, but millions of gamers use its Steam service to buy and manage their library of games.
Steam began as a way for people to manage games on PCs but Valve has been working to expand its reach.
Steam has a Big Picture mode that makes it easier for games to be played on big screens.
In addition, it is known to be working on a Steam box, a console that gives people access to their account and lets them play games on a TV.
Rob Crossley, writing on the CVG games news site, said family sharing was "likely to now be a key feature of the long-awaited Steam box".
Also, he pointed out, the sharing idea was very similar to one Microsoft floated for its forthcoming Xbox One console.
The idea has now been withdrawn but Microsoft has hinted that it might return.