Gilder Lehrman Institute of American History
Gilder Lehrman Institute: The Columbian Exchange
[Free Registration/Login Required] The story of the Columbian exchange is a story of two worlds colliding. The Europeans brought horses and wheat, they also brought germs. Here's a synopsis of the effects of that fateful encounter.
BBC
Bbc: History: Columbian Exchange
A page describing the Columbian Exchange lists the articles of the exchange and the wider scope of the impact on the world.