This was compiled in the Macworld OS X Forum, and I thought it may be of some use to everyone. If you'd like to replace the stock Apple toolbar icons with your own, here's how to do it.
- Find the icon you want and then launch Icongrapher, use the Open Icon command to open the icon and then pick "Save as" a .icns file.
- Log in as the root user [Editor: You could also use the Terminal to work around having to login as root]. In the Finder, navigate to /System/Library/Core Services/ and then control-click on the Finder icon and select "Show Package Contents".
- Navigate through the "Contents" folder to the "Resources" folder. You will see all the icons easily named (computer.icns, delete.icns, etc).
- Remove the file of the icon you want to change [Editor: A better suggestion may be to rename the original file in case you want it back!], and replace it with the one made by Icongrapher.
- Log out and login, and VOILA, new toolbar icons! [Editor: You could also force quit the Finder to make it restart and read your new icon files, although logging out and in is certainly a cleaner way to do this!]