Mode for Garry's Mod adds the Riverside map from the Left 4 Dead finale.
Map for Sandbox mode - a huge city where you can do anything, and a separate track for races.
Big shelter. The author was inspired by the game Fallout 3. Various rooms, suitable for the most diverse modes. No CS:S is required to launch.
A small map with a mini-game that transports you to the training field from Half-Life 2.