[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.
[GTK-server] (5.0/5.1) - GTK binding using the GTK-server. Can use GTK1.x, GTK2.x or XForms. Wrapper to make the interface more friendly is [here] - needs gtk-server 2.1.5.
[LuaGnome] (previously called lua-gtk) (5.1) - A fairly complete binding to Gnome libraries, including GTK2.
[lgob] (5.1) - Bindings for [GObject based] libraries, such as GTK+ 3 and many others (cairo, gdk, gstreamer, pango, webkit, ...).
[tekUI] (5.1) - small, lightweight graphical user interface toolkit; not a binding, but written in Lua itself. Available for X11, DirectFB, Windows and others. Extensible and highly customizable. It's very clean and lightweight but is still in an early stage. For example, no multiline input field, clipboard and context menu support.