SourceForts is a Half Life 2: Deathmatch mod where Combine and Rebels face off in capture-the-flag action with a big twist. Players build bases and other structures with blocks and plates in the "build phase" to protect themselves and their team's flag (which is actually a ball), and then duke it out and try to capture the enemy's flag in the "combat phase". Once the combat phase has ended, it's back to build, where players can repair, improve, and change their creations to ready them for the next combat phase.
This game like any other had it's problems. Griefers by the bucketload, occasional crashes and disconnects...
But you keep playing it. And playing it. Because when you find that one decent game of the night. One game where both teams work well together and no griefers appear... This is beyond anything else out there in the sheer amount of FUN you will have playing it.