libvips is a demand-driven, horizontally threaded image processing library.

It implements around 300 operations covering arithmetic, histograms,
convolution, morphological operations, frequency filtering, color,
resampling, statistics, and others.

• libvips @ GitHub.

A lightweight, simple, fast, feature-filled, and extremely extensible text editor written in C, and Lua, adapted from lite.

Lite XL @ GitHub.

Scientific computing for LuaJIT.
Torch is a scientific computing framework with wide support for machine learning algorithms that puts GPUs first. It is easy to use and efficient, thanks to an easy and fast scripting language, LuaJIT, and an underlying C/CUDA implementation.

As you probably know by now, LÖVE is a framework for making 2D games in the Lua programming language. LÖVE is totally free, and can be used in anything from friendly open-source hobby projects, to evil, closed-source commercial ones.