Half Life Mods.
Xash3D is a really good way to play older half life mods and new ones
DownloadEntropy : Zero 2 is the absolute best half life 2 mod ive ever played, about 7 hours of content thats fully voice acted and beautifully made. this is only the demo but the full version is on steam, if you own half life 2 you have to download it.
They Hunger is an amazing three chapter half life mod set in 1950s America. The un-named character you play as is a writer who recently moved to the town to inspire his next book, but after his car is forced off the road and into a lake the protaganist discovers the town is over run by zombies. good mod thought the sequels are less serious and do not have the same quality of the original.
Half-Life: Echoes