Indeed, the Inca empire (Tawantinsuyu) was one of the fastest growing, and later, fastest declining in the world. It came to an abrupt halt when the Spanish conquistadore, Francisco Pizzaro invaded and captured the Emperor, Atahuallpa. The efficient and rigid hierarchial bureaucracy of the Incas completely broke down without an Emperor to give his consent to any major descision.
The famous 'knotted strings', known as kipu (usually spelled quipu as it went through Spanish) were mainly used for record keeping. They could only represent numbers and sums -- many knots close together represented ones digits, and a different know tens. Sometimes runners would carry a handful of yarn.
The Inca language, Quechua is the most spoken Amerindian language today, and is used as a lingua franca by the 13 million people (mostly in Chile) who speak it. Like many Amerindian languages, it is heavily agglutinating, like Esperanto. What is especially amazing is that such a great empire was establised without the use of the wheel. In addition, the Incas built the greatest road system of any pre-industrial civilisation except the Romans | 
