Serial.Cloud - the Serial Killers Database, an ongoing research project which aims to sort and classify serial killers based on documented references from books written about serial killers as well as other online resources listed at the bottom of each killers profile page. The database currently contains 622 serial killers that are classified into serial killer types. Serial killer types are based on acts committed during a killers serial spree such as: torture, sexual assault rape, stalking, strangulation, consumption of human flesh or organs cannibalism, consumption of human blood vampirism, and/or sexual assault after death necrophilia. Killers can belong to more than one serial killer type.

Radford / FGCU Serial Killer Database -  research project based at Florida Gulf Coast University.  In 1992, Dr. Mike Aamodt started to collect and code data on serial killers. Today, the database is the largest non-governmental serial murderer database in the world. It contains 3304 subjects, comprising of serial killers, mass murderers, spree killers and more. The database has over 9000 victim profiles and over 500 documents. but is no longer available to the public. Annual reports are compiled by FGCU professors and available on ReasearchGate, a free website for scholarly research.

Murderpedia - the encyclopedia of murderers  - Murderpedia is a free online encyclopedic dictionary murderers and the largest database of serial killers and mass murderers around the world.

Crime Museum - Crime Library. The Crime Museum is an educational resource on law enforcement, crime, and forensic history, and supports museum and education related exhibits and programming. Crime Museum operates the Natalee Holloway Resource Center, the Crime Library, with over 2.5 million annual pageviews, and is a repository for famous crime and law enforcement related objects.  Its Crime Library contains profiles of many notorious serial killers.

Biography.com - Infamous Serial Killers  - Every life has a story. Biography.com captures the most gripping, surprising and fascinating stories about famous people. The web site has created a page detailing the lives and crimes of the more notorious serial killers.

National Criminal Justice Reference Service - enter "Serial Killers" in the search box   - search results list materials found or referenced throughout the site on the subject of "serial killers."

Wikipedia  - has three articles with data on serial killers. 1) List of Serial Killers by Country - -notable criminals, listed by country where they committed the most murders  2) List of Serial Killers in the United States  3) List of serial killers by number of victims - does not include killers only active before 1900.  Be aware that these may not be up to date and references may not be complete.