Urho3D
A lean, MIT-licensed C++ 3D engine from Finland — the serious option for people who wanted a real engine but not an entire platform.
Finnish developer Lasse Öörni built Urho3D as a compact engine drawing on OGRE and Horde3D: rendering, audio, networking, a component-based scene graph, physics via Bullet and navigation via Recast. Game logic goes in C++, AngelScript or Lua, and the same code builds for Windows, Linux, macOS, Android, iOS and the web.
Through the 2010s it was a popular pick for those who found Unity too closed and Irrlicht too dated. Around 2022 its small community split, and development now carries on in forks such as rbfx — a common fate for open engines with no company behind them.