Put it simply Azure Sheep is what Blue Shift should have been
It's been made before Blue Shift was out, it's as long as Opposing Force (which means there are quite a couple hours to spend), it's well designed, right in the style of the Half-Life levels. Of course the story takes place in Black Mesa... but the end levels are REALLY well made, when you fight at the surface in the ruins of a city against apaches, tanks, militaries and special forces... but I don't want to spoil it. Give it a spin, it's worth the download
http://halflife.multiplayer.it/azuresheep/
And you definitely want to play They Hunger 3, it's the BEST one in the trilogy !!! Graphic details, ambience and storyline altogether !
http://www.planethalflife.com/manke/theyhunger.htm
@elitejavi: The Specialists is a multiplayer mod (and the best one). It cannot be played in single player. Check out
http://www.specialistsmod.net