Research and Development (HL2:EP2)
I can't really give this one a finished number because it's just a Half-Life 2 mod but it's a REALLY GOOD Half-Life 2 mod.
Anyway, Research and Development. Requires Half-Life 2: Episode 2, which you probably already own because it was part of The Orange Box and god knows everyone's played Portal and some of us enjoyed TF2. This is almost a puzzle mod, much like Portal, except there's quite a bit of action to it. There's no weapons except for the environment, the gravity gun, and the antlion pheromone pod. It starts off with some pretty basic physics puzzles, then it starts to get into WTF physics mode, and by the end you're doing some really elaborate tricks. It's Portal-short in length, but that's not a bad thing. It's absolutely enjoyable without overstaying it's welcome.
I'm not going to throw out any spoilers here, even though there's not much of a plot, because there's some pretty good gags. With a couple more levels and some spit-shine, I could see this being included in the next Valve release if it weren't so vanilla Half-Life. I mean, this is fun and well done but there's nothing that really differentiates it from the other Half-Life games like TF 2 and Portal did. You can't look at a Half-Life level 2 and confuse it for Portal. As it is, any of the levels in R&D could be dropped into Half-Life 2 and no one would know the difference.
Don't let my negativity about it getting picked up by Valve stop you though. If you own Half-Life 2: Episode 2, you should play this. It's pretty awesome.