Crimson editor is a cool editor that is free. It does source highlighting of MEL if you download the proper files from this site. I love the ability to edit files over ftp as if they were local. Great for my network programming work.
I tried the Melanie application because it purported to have MEL interface design - but I ended using it's color coded editor instead. It seemed to be more intelligent about highlighting than Crimson Editor ( because it was made for MEL I suppose.)
But I do most of my coding in Microsuck Visual C++ and it ended up being convenient to edit there as well because I keep my scripts in source safe - and it's all integrated. So there are highlighting files for this as well - but it's not free.
Also, it ended up being an issue to me that the editor have easy/fast ablity to jump a given line number because the MEL errors have useful line numbers. So I dropped Melanie for the reason. Crimson Editor and MSVC are both fine for that.
my 2 cents.
-Tawn