Kharosthi was used to write Sanskrit and Gandhari about 2000 years ago, and is the script of the oldest Buddhist manuscripts yet found. It plays a role in Eliot Pattison's mystery "Water Touching Stone," which I am currently reading. An example of the kind of wooden tablet mentioned in that book can be seen here.
Two fonts which include Kharosthi are Alphabetum Unicode and MPH 2B Damase. A rudimentary keyboard layout can be downloaded from my iDisk.
The available fonts and OS X are not yet capable of displaying Kharosthi properly.