I just found an Esperanto keyboard layout for OS X that seems to work well in 10.5 (Leopard).
It's available here:
http://www.options4.com/options/software/keyboards.xhtml The most recent version, 10.3, is the one I chose. I used Pacifist to extract the .bundle file from the package and put it into "~/Library/Keyboard Layouts", since I prefer to do things manually. Thus, I can't guarantee that the installer itself works.
And if you're not familiar with Esperanto, I recommend reading this:
http://esperanto.net/info/index_en.html
