[wxLua] (5.1) - [wxWidgets] bindings for Lua 5.1. wxLua project provides its own IDE with a GUI debugger, a binding generator and a "wx" module for wxWidgets. The IDE is written in wxLua. wxWidgets provides native look-and-feel on Windows, Unix and Macintosh platforms wrapping the underlying toolkit (respectively win32 API, GTK+ 2.0 or Motif or X11, and Carbon API). Well-maintained.
[IUP] (5.0, 5.1) - GUI toolkit for Windows, GTK and Motif with Lua 5 bindings. IUP uses native controls in Windows, just like wxWidgets. The Lua binding has a very close Lua like API.
[VCLua] (5.1) - Lua Visual Controls, small native toolkit library for Windows and Linux/GTK2.
[Crank Storyboard Suite] (5.1) - Create and deploy user interfaces to a wide variety of operating systems (QNX, WinCE, Windows, Linux, Mac, ...) and rendering technologies (gdi, sdl, opengl, openvg, agtdk, direcfb, fbdev, ...) using standard graphic elements and Lua scripting to manage the glue logic. Also provides direct from Photoshop screen import capabilities and an integrated simulator and Lua debugger.
JUCE: [luce] (5.1/5.2/5.3) - lightweight, embeddable GUI toolkit based on JUCE.
[tekUI] (5.1/5.2/5.3) - lightweight, modular, extensible graphical user interface toolkit; not a binding, but written in Lua itself. Available for X11, DirectFB, Windows, Linux framebuffer, raw memory. Customizable using cascading style sheets. Option for an inbuilt VNC server.
[lwtk] (5.1-5.4) - Lua Widget Toolkit - a foundation for building cross platform GUI widgets in pure Lua on top of [lpugl].
TUI (UI for terminals without graphic capability)
Termfx [LuaRocks and https://tset.de/termfx/index.html] (5.1/5.2/5/3) - TermFX is a module that a program written in lua can use to create text based user interfaces. At its core it uses termbox, but provides a more lua like API, and some additional stuff like offscreen buffers and xterm color management. Alternative to curses and much easier to learn and use; note though that relies on termbox library which is included in the installation but no longer maintained--may need to build instead with an updated fork from termbox on GitHub?.
LibrariesAndBindings - lists other graphics libraries for OpenGL/3D, direct media access, images, plots/graphs, etc.