In the February 2007 issue of Games for Windows: The Official Magazine, Arkane Studios unveiled The Crossing — the world's first "cross-play" PC shooter, set in Paris in multiple eras and spanning dimensions.
So what's cross-play? Here's the crib notes version for those who haven't had a chance to read GFW's cover story:
In the simplest sense, story missions are single-player shooting with an exception: Naturally intelligent human opponents take the place of A.I. There are two types of players: Elites and skirmishers. Elites are gamers playing the game in story mode. They're beefed up, heavily armored, and heavily armed. They have to be able to hold their own against a swarm of skirmish players. Skirmishers are gamers who typically play on multiplayer maps: well-trained, rank-and-file soldiers playing primarily to have some quick fun and increase their rank through defeating the occasional Elite. Skirmish players can also invade story maps and "possess" A.I.s ala Agent Smith in The Matrix.
Still confused? Producer RaphaŽl Colantonio and art director Viktor Antonov explain further.