Valve's Half-life is one of the best games, and one of the best game series, to grace the Steam library. It was revolutionary, and not only proved that mindless shooters can have a compelling story.
Half-life 2, proved that good physics can be a vital asset to shooters/ With the inclusion of the
Gravity gun, one of the series most iconic weapons, the Source
physics engine is put to the test, being able to launch entire boxes, and barrels, and throw them an insane distance.
The music is an integral part of the series. It puts some extra flavor on everything. Like when you come upon a very eerie setting like
Ravenholm, creepy music begins. Or when you get thrown into a
HECU ambush, or a Hunter-Chopper chase, amazing
action packed music starts playing.
Something that helped take off the series was the inclusion of a
silent protagonist, Gordon Freeman. Giving the player a character that only makes decisions that the player makes, means that you don't get to see a story though the character's eyes, you are the character. This stays pretty much the same through out the entire series. And not to mention that so many things came out of these games. Such as Counter-strike, Team Fortress 2, or even Garry's Mod, even though it's not a Valve game, it was built directly off of Half-Life 2.
And I can go on for hours, but you've probably got better things to be doing, like school work. But I don't blame you. School is terrible.
Long story short: Half-life is fucking amazing.