Unicode 5, released August 2006, includes 5 new scripts. Here is the current status of their usability on the Mac as far as I know. Where there is a font but no keyboard, the characters can of course still be entered directly from the Character Palette.
Fonts: ALPHABETUM Unicode, Code2001 and MPH 2B Damase
Fonts: (Hittite glyphs) FreeIdg