Nuclear Winter (HL mod)
Some bonehead alliance (Russia, China, Arab countries etc.) has invaded the US. Major cities have been nuked and foreign troops have landed on the US soil. Your job is to kill the bad guys and save the free world, or what's left of it.
The add-on supports Steam and no longer requires Opposing Force to run. You play the role of a US soldier, Corporal Adrian Shepherd. This 29-level add-on takes you from wastelands to big cities and to a nuclear missile silo.
Levels are well designed and I just loved the flow. There are enough different locations to keep you interested. You start from the wastelands of Rosswell. This part consists of mostly outdoor areas with a few hi-tech parts (for example, a dam system). From there you'll proceed towards a somewhat abandoned city where you have to fight some nasty "radiated animals" (normal HL/Opp.Force aliens). The finale takes place in a nuclear silo.
Levels are pretty easy. The fact that you're given the best weapons of the game (Sniper rifle & SAW) near the beginning tends to help. Combat is easy but fun - thanks to Opp.Force guns and well balanced combats. Puzzles are rather easy for the most part but I got stuck a few times and had to use a walkthrough that's available at Nuclear Winter website.
Author uses a good amount of scripted sequences to spice up the gameplay. A few guitar melodies can be heard at various locations and they fit quite well, providing a sense of a post-apocalyptic world (remember "Don't Fear the Reaper" from The Stand?). A few of your fellow troops follow you every now and then, providing assistance and cover.
One of the best Half-Life/Opp.Force mods. The levels look good and the gameplay flows nicely through the game's post-apocalyptic world.
Website: http://www.planetquake.com/lm/
Download is 22Mb.