Four years ago Congress began requiring states to maintain sex offender registries to track the whereabouts of criminals once they leave prison.  States around the country are now going a step further, posting that information, along with photographs and sordid details of crimes, on Web pages accessible to anyone with a computer and a modem.  According to the New York Times, proponents of the trend, including many state officials, say expanding community notification to the Internet is a valuable and relatively simple way to disseminate widely information on dangerous sex offenders.  Before there were online sex offender registries, anyone wanting information of specific offenders had to request it from local or state police agencies.  That is still the method in New York and California.  Proponents say moving those sex offender registries online is consistent with the intent of the law that created them, making it easier to make the public aware of sex offenders in their communities.  But many experts say no one has adequately thought through the consequences of the online registries.  One worry is that the registries have the potential to lead to retributive violence against offenders.  As notification laws become ubiquitous, so have incidents in which ex-offenders were harassed by neighbors, evicted by landlords, fired from new jobs or beaten by revenge-minded mobs.  The Parents for Megan's Law Web site provides links to sex offender registries nationwide at