GTK Quartz Engine
One of the nicer features of GTK+-2 is its theming facility, an easy way to make an application look the way you want it to.
Most Mac users would rather have their GTK+-2 apps look like native Quartz apps instead of like Motif apps. There are several ways to get there, but the one we support is Gtk-Quartz-Engine.
Gtk-Quartz-Engine has an optional module provided in the gtk-osx-build modulesets, so to install it all you have to do is
$ jhbuild build gtk-quartz-engine
and either adjust the .gtkrc-2.0 file in your home directory or, if you're using a bundle, Contents/Resources/etc/gtk-2.0/gtkrc-2.0. There's a sample gtkrc-2.0 in the gtk-quartz-engine distribution, and the bundler example includes the transfer element.
A little work has been done on the engine since Richard Hult withdrew, by Rob Caelers. If you want to work on it, fork Gtk-Quartz-Engine and let us know about your changes.
There are still some rough edges, so any work is welcome!