Base Defense is a single/cooperative modification based on horde-style and RPG systems. It's awesome, you should try it ;)
Great mod, really made the traditional Half-life mechanics feel modern in-tune with the Horde-style systems we see in most shooters and stays fun throughout. Alot of balance problems though, most noticeably the AI-pathfinding which is so horrendous at finding players hiding in spots that its game-breaking at times. However with 4 players running around the map proper, its a blast.