My day job involves looking after large geotechnical and mining applications for the CSIR, South Africa. I have recently got the hang of editing Wiki pages properly ;)
The SciTE programming editor  is powerful, compact and scriptable with Lua. I have contributed several useful scripts (see in SciteScripts) including SciteWordSubstitution, SciteMacroExpander, SciteBufferSwitch, SciteTags and SciteNumberedBookmarks.
SciteExtMan is a proposed script manager, which simplifies deployment of scripts in SciTE. SciteOther is a small DLL (for Windows only) which works around some limitations of the existing Lua bindings. For instance, one can control another instance of SciTE using the Extension Interface.
Currently I'm working on a general GUI add-on for creating simple dialogs in Lua.
On the pure Lua front, GenericInputAlgorithms shows how simple STL-style input iterators can make processing files easier. SimpleLuaClasses describes a scheme for making classic object-orientation easier in Lua. PointAndComplex gives useful
Complex classes, and also demonstrates how to redefine arithmetic operators for user-defined types.
PenlightLibrary is yet another attempt at a general-purpose set of libraries for Lua.
SequenceAdapters shows how the Penlight concept of sequences can be wrapped up, allowing for a pipeline of methods applied to sequences.
SymbolicDifferentiation shows that LISP people don't have all the fun when it comes to symbolic algebra.
(Mention unofficial FAQ and UnofficialFaqDiscussion.)