Half-Life: Blue Shift Summary
Sven Co-op turns the original Half-Life campaign, as well as others, into a playable co-op experience. Valve is so impressed they're supporting a full standalone version to release over Steam.
That means you wouldn't need to buy Half-Life or it's add-ons in order to enjoy the Sven Co-op mod, which is currently at version 4.8. They get total control over the engine.
Before Half-Life cemented PC developer Valve in most gamers' hearts forever, Gabe Newell reveals they had plans to release a submarine title.
The boss says that no idea 'ever really dies' at Valve, it's more a question of "how long until we get around" to it. It's their "furthest back" unshipped project.