The game that made FPS games what they are today.
Graphics: A + for the time, they look crisp and enjoyable still today, and as the official gamespot review says "There is only 4 different types of professors" This is true, but it didn't make the game any worse.
Sound: One of the best sound design i've ever heard i a videogame! Weapon sounds? Check.
Voice acting? Check.
Soundtrack? Damn right!
Community: There are still players online if you wanna go try the robust multiplayer deathmatch of Half-Life, BUT, Half-Life spawned enormous amount of modifications, some of they are legendary to this day.
Modifications: As i said, the modifications for the game carry a damn good names on the list, here you have some.
Counter-Strike (The game is still one of the most played games at date, go check it out now!)
Team Fortress Classic
Deathmatch Classic
Day of Defeat
Ricochet
They Hunger
And much more.
The game also spawned 2 expansions that were more or less good, called Opposing Force and Blue Shift.