CyberTipline, maintained by the National Center for Missing & Exploited Children:

The Congressionally mandated CyberTipline, maintained by the National Center for Missing & Exploited Children, is a reporting mechanism for cases of child sexual exploitation including child pornography, online enticement of children for sex acts, molestation of children outside the family, sex tourism of children, child victims of prostitution, and unsolicited obscene material sent to a child. 

Reports may be made 24-hours per day, 7 days per week online at www.cybertipline.com or by calling 1-800-843-5678.