SciTE -
http://www.scintilla.org/SciTE.html
Cross-platform (well, Linux and Windows) but GUI-based, surprisingly configurable, small, free.
Doesn't require installation, files in system directories, or registry settings.
Drawback: hand-editing the configuration file can be a little intimidating to start with, and you'll probably want to tweak the default confiuration.
I usually have a copy on every PC I support, and my USB drive.
The exe-only version fits on a floppy with plenty of room to spare.
Code folding (yay!), configurable colour syntax-highlighting for a few dozen programming languages (including Lua), Lua scripting built-in, sessions (can work like projects), tabs, function-argument syntax-tips (if you have/create an API file), symbol/word auto-completion, library lookup (with an API file), regular expression search-and-replace, compiler/interpreter output capture window, brace matching/highlighting/selection, customisable helpfile lookup, etc.
Regards, Myles.