Favorite Source Mods?

[mirror's edgy]

I've recently obtained a fancy, cool (read: not worthless) laptop capable of running quite a few more games than my previous one, and I've just recently played a few notably Half Life 2 mods. I tried The Stanley Parable on Extra Credits' recommendation and thought it was very clever. I'm now about to try the psychological horror themed Dear Esther, which I've also heard praise for from EC. I'm instinctively interested in anything based on the Half Life 2 engine, and I like to see what creative little indie projects crop up online.

So what are your favorite mods made with Source?

 

[smearyllama]

GMOD, Zombie Panic, and Zombie Master come to mind.

Zombie panic is essentially a poor man's Left 4 Dead with some interesting differences, such as the fact that the horde is all players, not AI.
Zombie Master is like Left 4 Dead, but where the Director is an actual player that controls the zombies RTS-style. Also extremely popular with the furry community for some bizarre reason.
...and GMOD pretty much speaks for itself. Sandbox mod where you can build anything from anything if you're patient enough.

 

[Giest4life]

Undherhell is frickin awesome. It has some horror elements along with plenty of satisfying shotgun-to-the-face moments.