I know TextWrangler shows the option, but it doesn’t actually do anything. So what I want is autocomplete, either with a key combination, or while I type.
Syntax coloring/highlighting is a requirement as well, though I know Xcode and TextWrangler have that.
You might want to use Lua Development Tools for this purpose. It has syntax coloring, plus it is doing smart autocomplete, depending on what your actual context is (required modules, …)
It is an Eclipse Technology project, and it is Open Source! (EPL license).