Base Defense is a single/cooperative modification based on horde-style and RPG systems. It's awesome, you should try it ;)
Base Defense brings really nice features to the traditional half-life, it shouldn't be ignored the fact that it was made by only one developer.
The horde system, plus the details that enhanced gameplay, weapons and enemies shows a nice quality, always trying to maintain the original half-life style on them, and the ost (which is Great) adds a nice feeling to the battle, for me, this deserves a 10/10.