A currently unfinished mod for half-life 2 that includes co-op and deathmatch maps, however cheats need to be enabled to get the real feeling of gameplay.
list of debug keys
(require "sv_cheats" to be set to 1)
pressing "i" gives all weapons and equipment
pressing "o" spawns an antlion
pressing "v" activates/deactivates no clipping mode, which allows you to freely fly around the map without brushes or entities getting in your way
credit to some users on gamebanana for some content...