They are primarily known for high-quality free game-dev-assets (2d, 3d, sprites, sounds) and Asset-Creation-Tools like AssetForge. I would guess that most (hobby-)game-devs have used their content for tutorials, prototypes and the like.
They're also a gamedev. In the beginning, their assets were leftover/reutilized from their games to help other gamedevs with placeholder assets. Now, that's kinda more of their thing, with them developing entire asset packs for aspiring gamedevs.
