Half Life 2 online co-op mod Obsidian Conflict began life in 2008, and it was last updated in 2013, when a Source engine upgrade broke it so badly that the team decided to move onto other projects. Now it's back, and it's getting a Steam release.
The mod lets you play both Half Life: Source and Half Life 2, including Episodes 1 and 2, with friends via online co-op. On its own, that'd be enough to have me excited, but the bulk of the mod comes in the form of custom maps, game modes and NPCs built in the Half Life 2 universe.
The custom bits range from the obvious—a co-op prison escape or holding a fort against waves of enemies—to modes that seem alien to the Half Life world, such as building up a farm with a friend or going fishing.
The mod also adds new weapons, including dual Uzis, a sniper rifle and the Tau Cannon from the original Half Life. Some of the game modes have custom NPCs, too, including merchants that will sell you weapons, and the features you'd expect in a co-op game—such as HUD waypoints and a button to call for a medic—are all baked in.
The team has a "fully-functioning" version of it up and running, and they're planning to release it on Steam at some point this year. Check out the video below to see what they've been working on, and head over the the ModDB page for more.