Traditional Mongolian is perhaps the most difficult script to display on a computer. Much like Arabic, Mongolian characters are connected and can have different forms in initial, medial, and final word positions. Plus it is written in vertical columns from left to right. Mongolian has some current usage in Mongolia and China, and was also employed for the Manchu language which was used (in addition to Chinese) for official documents of the Qing dynasty in China during 1644-1912.
To do this script on OS X you will need to download an AAT font and a keyboard layout from the mongolfont.com site.
Here are some sites where you can see this script in action with Safari once you have the font installed. Thanks to Greg Pringle for these:
Birds of Mongolia
Inputting Mongolian Script