Bluefish is a code editor with a focus towards web development. Bluefish is open source software, and is supported on Linux, Unix, Windows and Mac OSX. There is a package for OSX 10.6 on the bluefish download server.
GRISBI is a personnal accounting application running under GNU/Linux and Windows, released under the GPL licence.
GRAMPS is a Free Software Project for Genealogy, offering a professional genealogy program, and a wiki open to all. It is a community project, created, developed and governed by genealogists.
They've packaged things up so that all you need to do is download a package and start working. Instructions here.
GnuCash is personal and small-business financial-accounting software, freely licensed under the GNU GPL and available for GNU/Linux, BSD, Solaris, Mac OSX and Microsoft Windows. There's a download link for the latest bundle on the webpage.
Gnaural is a popular Binaural Beat generator licensed under the GNU GPL and available for GNU/Linux, Mac OS X and Microsoft Windows. The downloadable application bundle is available here. That should work for OSX 10.5 (Leopard) or higher; a bundle for 10.4 (Tiger) is available here.
GIMP is the preferred raster image editor for GTK+ and runs on Linux, Windows and Mac. You can download the Mac version from here.