I use Movable Type. I really like the Syntax Highlighting option in the (otherwise simple textarea) editor.
Description and Documentation
I'm not a fan of WYSI(a)WYG. But I do like Syntax highlighting. I especially like it when syntax highlighting helps highlight bugs (e.g. a missing " may cause a chunk of code to run away in blue).
Movable Type (the weblogging system) has a nice syntax highlighting feature. It occurred to me that TWiki could benefit from something like this.
Examples
Screenshot of syntax highlighting in Movable Type: