a long, LONG time ago I was contemplating on ripping out that Plain Jane text editor in the admincp and replace it with TinyMCE or something cool like that. Then I merged that into my custom admincp idea.
But if you want an external solution, EditPlus is easy enough to write a language reference script for.
I started experimenting with Notepad++'s language styler when I first got it, and then realized it was probably going to be more trouble than it's worth. I guess no one delves into large template files enough to have already written one by now. What does EditPlus have along those lines? Like a built in styler?