ManDrake
ManDrake is a native, open-source man page editor for Mac OS X. It has syntax highlighting and live-updating rendered preview.
Long story short, I was sick of writing man pages in an endless cycle of edit-view, edit-view so I decided to do something about it and made this editor. It's a bit rough around the edges, but it works really well for me.
The current version of ManDrake is 2.1. It requires Mac OS X 10.6 or later, and is an Intel-only fat binary (32/64-bit). Source code is distributed under the GNU General Public License v2.
For the old version that runs on PPC macs (and Mac OS X 10.4 or later), you should download this binary (source).