Base Defense is a single/cooperative mod based on horde-style system. 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.
though the mod has some bugs, most notably the ai-pathfinding, it is one of the best co-op half life mods in my opinion. Also the game needs more maps, more quests and new weapons. graphics are ok, though it could have some better models/textures, but in the end gameplay matters(at least for me) and the gameplay of this mod is superb i think and really really fun!