I tried out Portal RTX, found the room where the light ball is casting shadows all around. It looked nice; but I also felt like I’ve seen the same effect imitated with regular rendering. Sure there might be slight differences, but I wouldn’t have spotted them.
Garry’s Mod. Basically a gateway drug to hobby animation, and in some ways not so far off from the modding tools used to make it.
If you’ve watched stuff like Heavy Is Dead, they’re usually made with it. Some more professional-looking stuff is instead made in “Source Filmmaker”.