First make sure that you installed mac ports. For this you also need to download the MacOSX SDK so that make etc. is setup properly on your system. Then you can choose a lot open source software and compile it easily from source yourself.
E.g. install gimp
The default X11 gtk theme looks very ugly, you can fix this by installing some themes (e.g. search with "port search gtk2"), e.g.
Then it's necessary to change it, use gtk-chtheme for this