A video maker using the half life engine. Like the other poster said, it's been used for a long time. This weird new series though has racked up 100s of millions of views and everyone is kinda scratching their heads a bit as to why

Source Film Maker, which uses the Source Engine, which Half-life and Team Fortress 2 are built on. It doesn't use bones in it's models, so animation is a lot more manual often leading to really cursed motion, but it's also quite easy to get running so lots of lots of amateur animation happens with it, particularly shitpposts and youtubepoops.