You've certainly seen their free magazines and perhaps even their emails. The Southern Poverty Law Center's education project is an outstanding site dealing with a variety of prejudice and reduction of prejudice articles, activities, and resources. One could literally teach an entire course utilizing the materials on this site.
Activities Search Page with a list
Mix It Up
If you are a believer in social justice, reducing prejudice, increasing understanding, and increasing an understanding of history, this is a fantastic site.