The form of writing we know as an "essay" developed as a way for an individual to express their opinion to wide audiences, and controversial topics have always been a part of essay writing - a far cry from how many students are taught to write "essays" in their school classes.
Check the news for topics which are spurring a lot of discussion, especially if it's heavily polarized. Has there been a crime spree in your neighborhood recently? How about discussions of children having sex while under age? These are both perfect triggers for controversial essays which will enlighten your audience as to your point of view.
Always make sure you have a strong argument that backs up your topic, no matter what controversy you have chosen to explore. Don't just say that something is good or bad, but back these opinions up with facts and figures.
Some items are almost always controversial, and in today's world some of the most controversial topics you can choose deal with: