Transmissions: Element 120 (HL2 mod)

Nice mod. The idea behind the 'new gun' is relly awesome; jumping around like you are ridden with fleas and red ants and only the wind can take them out of your flesh.

...Strange metaphor aside, it has horror, but it's mostly an action mod.

Definitely an action game instead of a horror game, but it's one of the best half life 2 mods I've played. Takes maybe an hour to get through. Only downside is it left me wanting more.

 It starts with a few elements of horror, but quickly becomes action oriented. It's short enough so it doesn't get boring, but it left me wanting for more.

This is a fun game! What starts as a tense puzzler ends in a explosion-loaded boss fight! Works for me!

If you're looking for more puzzles, try Research and Development (Here's what it looks like)

If you're looking for more shooting, try Entropy: Zero (Half the mod is showcased here)

I can personally recommend each of these three mods as short  (1-4 hour) endorphin-tickling good times.