To be fair, Garry's Mod is up there too.

The amount of customisation that one "small project" opened up in general, is astonishing.

Portal is basically a Half-Life 2 mod. It’s still got the fanboat asset. You can use the console to bind spawning a fanboat to an input, so there’s a speedrun category where you’re not allowed to use portals, but younare allowed to spam fanboats. Literally called fanboat%.

Speaking of Half-Life mods, Entropy Zero 2 is a solid game.

There’s also some great Portal 2 mods out there as well.

Endless hours playing Sven Co-Op. A few clips of my friends and my own earliest YouTube videos ended up in the NoClip documentary about Half Life!