Godot

An excellent piece of free software. MIT Licensed, and allows you to edit the engine source code. As far as 2D game engines go, I am willing to say it is best-in-class, though you may want to look into Gamemaker if you're on Windows, or Love2D as both are still decent options. The 3D capabilities in my opinion are about on par with Unity.

The scene flow, and the concept of resources makes development a fair breeze. The scripting language, GDScript is like Python 3, without any of the package management nightmares, and with a few language differences like Match statements and some engine-specific differences. If you need performance you can write C++ and C# code still.

It has a pretty outspoken community, and a few bad eggs in the community stir the political hornets nest. To me, this is fine as the software is still excellent, thought I try to keep my distance from that aspect of Godot.

Godot Addons

Godot has a large asset library that can help with some things that are more complex or more boilerplate work that is needed in all sorts of games, like main menus, terrain systems, level editors etc. Almost all of these are free and under an MIT license like Godot making them very useful.

There is also a complementary website focused around shaders in Godot, with many being MIT or Public Domain shaders.

=> Godot Asset Library
=> Godot shaders

gd_planar_reflections

A script I wrote for planar reflections in Godot. Due to the lack of clipping options within the Camera in Godot, it's an imperfect solution as can be seen in a github issue. I would recommend looking at a different planar reflection implementation nowadays, as there are a few available in the asset library now.

=> On the assetlibrary

func_godot

An excellent free addon for Godot that allows you to make levels using Trenchbroom or some other Quake level editors, and with it, you can then easily make 3D levels for Godot as Godot leaves a lot to be desired for level editing.

I would recommend getting it from github, it is frequently outdated on the asset library. I used this for Agora Runes and for Mirrormind.

=> On github

func_godot_lightpass

A complementary addon to go with func_godot. On its own it does nothing, but allows you to define things for Godot editor to export back into the .map file for easy positioning inside of Trenchbroom. This works extremely well for things that only render well within Godot, or for complex datastructures that only work within Godot and would be a nightmare to edit inside of Trenchbroom.

=> On Github

Terrain3D

An excellent clipmap based hand crafted terrain editor. I have previously used this for my game jam game Agora Runes.

=> On github