This is a link every parent should have bookmarked. The Family WatchDog is a national sex offender registry. It has a search engine that will display all known sex offenders on a localized map. Just type in a nearby street, your city and zip code. You'll be surprised at how many live near you.
It also has an option to search by names, so you can check on people who may have contact with your children.
Go explore the site and, if you have children, consider using some of the great features the organization offers, such as free notification when a sex offender moves into your area.