John A. Fotheringham presents data in tabular form on the robots sent by search engines and other sites to read and index Web pages: their origins, names and IP addresses.

A list from PGTS of Web robots with the identifying data they leave in Web site logs.

Search Tools Consulting explains how the search engine programs called "robots" or "spiders" work, and reviews related sites.

José Luis Pellicer's searchable database of robots, spiders and other user agents for programs that surf the web.

Lists IP addresses of search engine spiders. Can be searched by IP address. Also links to resources on spiders.

Andreas Staeding's large list of search engine spiders, similar Web robots, and Web browsers: their web-log identification and links to their originators.

An alphabetical list of user agents and the deployer behind them, compiled by Christoph Rüegg.

Tracks and displays real time hits and IP addresses of MSNbot, Yahoo's Slurp, and Googlebot. Provides aggregate hits of each spider over the last 30 days.

Brian Dunnintg provides a list of all the major search engine robot IP addresses, by full class C only.

This large database lists user agents in categories and distinguishes between robots and browsers.