Dawn of the Dead: Source mod for Half-Life 2

Originally inspired by the fact that you could assign tons of bots to go against you in CS:S, Dawn of the Dead was
created for intense, action-packed, non-stop slaughter of
zombies. Using the name from the movie, Dawn of the
Dead pits you against as many zombies as you can handle.
Buy ammunition, night vision, and grenades as you prepare
for the onslaught of merciless zombies who want you dead.

Cower in fear as you walk through near-pitchblack maps
with nothing but a flashlight for a light source. It is the
only way you have a chance against the enemy. They are
everywhere, ready to jump out and tear you apart the first
chance they get. Mess your pants as they learn how to
send some of their numbers as bait so that you empty your
clip into the last zombie on screen. Watch as that zombie
falls with a satisfying thump onto the pile of bodies you
created. Then panic as you try to reload while more
zombies rush in and tear you limb from limb.

Team up with a partner to endure the chaos as you watch
each others backs and travel through the maps keeping
your ears focused as you listen for footsteps of the
approaching herd. Plan out your attack so you drag
zomies into an area where you can mow them down. Try
your hardest to prevent yourselves from being cornered as
they surround you to each get a piece of your unspoiled
flesh.

Create blockades and obstacles in an attempt to slow down
the quickly approaching mass of the undead spawn. Move
couches infont of doorways to slow their advancing
movement. Turn tables over for cover as they effortlessly
break it into shards of cheap wood.

but most of all...

run in fear, because they'll eventually get you...