Godot Engine - Feature-packed 2D and 3D open source game engine.
Damn Good Tools - Easy-to-use, fun productivity tools—free (and open-source).
GDevelop - GDevelop is an open-source game making software designed to be used by everyone.
Tool Finder - The wikipedia for 100+ productivity tools/apps
Unreal Engine - Unreal Engine 4 is a suite of integrated tools for game developers to design and build games, simulations, and visualizations.
IT Tools - IT Tools is a free and open-source collection of handy online tools for developers & people working in IT.