Obsidian Conflict

Obsidian Conflict is a free cooperative multiplayer Source Engine game set in the Half-Life 2 universe.
It features a large amount of new gameplay features on all-new maps specially designed for Obsidian Conflict with focus on cooperative gameplay, as well as over hundred user-made maps following the same formula.

In addition to that, you’re able to play through vanilla Half-Life: Source all the way up to Episode 2.