Sven Co-Op isn't just any Half-life mod. It's one of the oldest and highest regarded mods for the game, with countless releases across 17 years of its existence, and now it's on Steam as a free standalone game.
Like its name suggest, Sven Co-Op let's players team up and play through the original Half-life campaign together. However, the Sven Co-Op also has a ton of custom maps that has players working together to accomplish special objectives, like fighting off hordes of enemies and solving puzzles. You can watch the trailer right here:
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Sven Co-Op is available on Steam right now and, again, it's standalone so you don't need Half-life to play it. You can grab it here or through the storefront in Steam itself.
I played some Sven Co-Op back when it was new and a few times after that over the years. It's always been a cool, wacky experience. Definitely worth checking out since it's free even if it's a bit hard to look at.