Catalyst

Ever wanted to create a theme for Mac OS X? Catalyst gives you all the tools you need to browse the in's and out's of Aqua and the interfaces of all your favorite Mac OS X applications. Best of all, you can create your own interface themes.

Download Catalyst 8.0pr3.

Gluestick

Gluestick is a simple yet powerful graphics editor exclusively for Mac OS X Leopard. Use Gluestick to quickly and easily create graphics compositions and icons.

Click here for more information.

ThemeKit

Have you ever wanted to write your own theme software using Apple's powerful Cocoa API? Now you can! ThemeKit is a Mac OS X framework for reading and writing .skin files, rendering Mac OS X gui widgets (and manipulating them), and writing out resource files built for the current version of Mac OS X. It's a universal framework, and it's open source!