The lead developer of Half Life: Crowbar-Deep in the Dead and Half-Life Decay: Solo Mission.
Has some bugs that need addressing, but is currently the best way to play Decay if you don't have a second player on hand
It's everything that you could want from a Half-Life mod. Levels with suspense, comedy, horror, puzzle-solving, it's like the entire Half-Life mods section rolled up into a single experience!