The really scary part is that the only ones who are registered are the ones who have been caught, convicted and required to register as part of their sentence. For every registered sex offender in your neighborhood there are probably 9 more walking around that no one knows about yet.
A couple of places to look to see whether you have any in your neighborhood:
http://www.sexcriminals.com/registration-notification/
http://www.klaaskids.org/pg-legmeg.htm