Read the original Co-Op Classics article
Joust is probably the first co-op title most people remember playing. Sure, there were earlier examples, but they weren't nearly as memorable as the original ostrich flight simulator. Cooperation was directly promoted on team work waves, where bonus points could be racked up if players avoided killing one another. But to advance very far at all, you'd need to work together on every wave. Watch out for that pterodactyl!