Ukelele

"Ukelele is a Unicode Keyboard Layout Editor for Mac OS X versions 10.2 and later. ... Ukelele is written by John Brownie of SIL and is copyright ©2003-2007. This software is provided under SIL's standard freeware license."

Webpage: "Ukelele: Mac OS X Keyboard Layout Editor" http://scripts.sil.org/ukelele