See Filtering Service on The Door - excellent sources for monitoring your children's access to the Internet.

Yahoo search on blocking and filtering -- annotated listing of software available online.

Cyber Snoop -- Instead of trying to sanitize the Internet, Cyber Snoop places the emphasis on teaching responsible Internet usage through supervision and communication.  Cyber Snoop is a tool that educates users to make positive choices when tapping the potential of the Internet.   Downloadable trial software available.

Net Nanny -- commercial software that acts as an invisible monitor between the Internet and your family.  It operates quietly in the background, carefully screening out user defined sites, words, phrases, and content that you have determined is inappropriate.

Triple Exposure -- a powerful computer search utility that offers a unique alternative to standard pornography filtering and blocking software.  

WizGuard -- powerful filtering capability is based on a content sensitive technology that focuses on the patterns of how all pornographic Web sites are put on the Internet.  WizGuard effectively blocks pornographic Web sites and does not block sites on sexual harassment, breast cancer, AIDS, safe sex, sexually transmitted diseases, unless those sites contain offensive content that are deemed inappropriate for children to view.