ETC2 is a single player mod made by DaveJ for Half Life. Set within the Half Life universe, it brings back Gordon Freeman to fight against aliens, zombies and soldiers.
ETC shows that even the older Half-Life mods can have great level design. It goes right along with USS Darkstar as my favourite mods because it can do so much with so little which is what Half-Life is about. While it lacks the puzzles it makes up for a fast pace, the place around that really feels like its about to blow, with every section feeling new and different.
ETC 2 carries even better level design, but this time it feels more like a story, with 2 characters you meet. Its so much like half-life also having to run from a large monster. Its the attention to details that make ETC one the best.