Grand Theft Auto 5. The game's online component lets up to 16 players enter Los Santos together, where they can take up jobs and missions, compete in races, customize characters, create content like races and deathmatches, and generally roam and tear asunder as they please.
In a news-wire update this week, the studio assured fans the confirmed presence of microtransactions "should not disrupt the playing field," since players will still need Reputation Points earned by play to be able to buy high-end gear; players can, however, trade real-world money for in-game GTA$.
Rockstar released new details on GTA Online this morning, including that it takes place a few months prior to the events of GTA 5. Also, house party invites are only for "friendly" visits, meaning other players can't tear up your pad - that's just good manners, really.