http://store.steampowered.com/app/222880/ ($14.99 USD base, $13.49 until Jan 29th, Four pack $40.49 until Jan 29th)
I didn't play the mod in its heyday, but it was one of the pillars of Source modding from waaaaaaay back.
Take to the streets for intense close quarters combat, where a team's survival depends upon securing crucial strongholds and destroying enemy supply in this multiplayer and cooperative Source Engine based experience. The follow-up game to the award-winning Source mod, Insurgency is highly competitive and unforgivingly lethal, striking a balance between one-life gameplay and prolonged action.
I didn't play the mod in its heyday, but it was one of the pillars of Source modding from waaaaaaay back.