Yi (or Lolo) is spoken by 4-5 million people in SW China and is written using 1165 characters representing individual syllables. For some interesting info, including lists of the syllables and sample text, see the Babelstone Yi Page.
The logical way to type Yi is with an input method like those used for pinyin Chinese. OS X includes a facility for creating custom IM's, so I made an experimental one for Yi. You type in the Latin letters for the syllable, hit return, and the right character is placed in your text.
To install this IM, download the file yi16.txt.dat from my iDisk. Then use the "Generate IM Plug-in" command in the Traditional Chinese input method menu, and select Yi in the latter. If you are using something earlier than Tiger, you follow a different procedure -- see this page for details.