Cross-platform 2D and 3D game engine
wxWidgets/Qt for desktop, SFML for 2D games. Haven't seen recommendations for 3D except things like Godot. But https://urho3d.github.io/ looks fancy.
NOTE: The number of mentions on this list indicates mentions on common posts plus user suggested alternatives. Hence, a higher number means a more popular project.
I'd like to learn game engine development - where to even start?
2 projects | reddit.com/r/gamedev | 30 May 2022
Game Engine Renderer Architecture regarding UI
1 project | reddit.com/r/gamedev | 26 Mar 2022
achieving 00's / ps2 graphics
1 project | reddit.com/r/gamedev | 25 Sep 2021
What code only engines for C or C++ are there?
1 project | reddit.com/r/gamedev | 8 Aug 2021
Best library for 3D development that doesn't require GLFW/GLAD etc
1 project | reddit.com/r/gamedev | 8 May 2021