Phags-pa is a script once used for Mongolian and Chinese and occasionally today for Tibetan. Andrew West has just produced a font for this which can be downloaded here. A keyboard layout for OS X which mirrors Andrew's version for Windows can be gotten from my iDisk.
Unfortunately as far as I can tell neither OS X nor X11 apps can yet display Phags-pa totally correctly, only Windows. Here is a test page.
Update Nov. 2007: OS X 10.5 Leopard can display Phags-pa correctly in Safari and Pages, but not in TextEdit.