I'm using SciTE, modified for WoW and Lua...from
WoWGuru:WoW-SciTE-Lua
It'll open basically any text file, but has special colors and junk for lua and xml and build in code-completion stuff just for WoW and WoW's Lua...and you can keep a bunch of pages open at the same time, to steal...err...examine other AddOns' code...
I'm just starting out, and it's making the job easier...