Kinda Moddable! Fan Patched! The Unofficial fan patch is a must to play. Based on the source engine so it is moddable but it is tricky. There are quite a few awesome mods for this on Moddb!

Fan Patched! While not very moddable outside of making maps it is fan patched! Turn off save gems and AI companions and the game is the true Quake 2!

Is a Mod! Map Editor! Steam Workshop support! what if portal had a gel gun and not a portal gun? has a map editor like Portal 2. Gameplay based off of the student project Tag the Power of Paint.

Mostly Moddable! Steam Workshop support! Make money from mods! Mod zeds don't seem to be a thing here unlike the first game. You can have cosmetic mods submitted to be added to the game. Custom maps!

Highly Moddable! Was a mod! Steam Workshop support! Want new zeds? new maps? new modes even? they are all there! Started life as a mod for UT 2004.

Highly Moddable! Steam Workshop support! The mod tools are built right in the game and can be accessed from the main menu!

Mostly Moddable! Almost all of the mods that started here moved to Arma 3 but there are still a few good ones like the fact that DayZ started here. Just get Arma 3 for the mods.

Highly Moddable! Steam Workshop Support! The mod tools for this game are insanely powerful. PUBG started here.

Highly Moddable! Steam Workshop support! So many good mods and I have even made a mod collection you can try out!

Is a Mod! A mod for Half-life 2 that is just bonkers good fun. Needs more Ninjas.