Do you mean after they have been caught and released from jail, or trying to catch them, like in chat rooms?
--Computer programs are really big--this one is called Cyber Sentinel. Here's two articles: http://query.nytimes.com/gst/fullpage.html?res=9D05E6D61730F932A05751C1A9679C8B63
http://www.securitysoft.com/press_release.asp?NewsID=11
And even LexisNexis helps: http://www.bizjournals.com/dayton/stories/2008/03/10/daily13.html
--Monitoring bracelets: http://findarticles.com/p/articles/mi_qn4191/is_20080221/ai_n24395049
--The government is getting in on it: http://gov.ca.gov/press-release/5485/
--And so are residents, who can actually VOTE on whether ot no sex offenders can live in their town (!) (or at least in this town--) http://www.concordmonitor.com/apps/pbcs.dll/article?AID=/20070305/REPOSITORY/703050343/1043/NEWS01
--Each state has a registry for offendors where each offendor must register. Every citizen can go on these sites and search a name to see if they have a sexual predator living near them. Here's Florida's site: http://offender.fdle.state.fl.us/offender/homepage.do;jsessionid=LH7pRVdcJGX22v5yWLt3G2l04c1PW7yTvCjxjRYGDYnwmxPSTPCX!2101905149
--States are using GPS: http://peopletrackusa.wordpress.com/2008/06/24/more-states-move-to-use-gps-tracking-of-sex-offenders/
--And then there's that show "To Catch a Predator"--http://en.wikipedia.org/wiki/To_Catch_a_Predator