All About Search Indexing Robots and Spiders
Search Tools Consulting explains how the search engine programs called "robots" or "spiders" work, and reviews related sites.
HTTP User Agent Index
An alphabetical list of user agents and the deployer behind them, compiled by Christoph Rüegg.
List of Robot Agent Strings
A list from PGTS of Web robots with the identifying data they leave in Web site logs.
Psychedelix List of User-Agents
Andreas Staeding's large list of search engine spiders, similar Web robots, and Web browsers: their web-log identification and links to their originators.
Robots, Spiders and Other User Agents: a Resource
José Luis Pellicer's searchable database of robots, spiders and other user agents for programs that surf the web.
Search Engine IP Addresses
Lists IP addresses of search engine spiders. Can be searched by IP address. Also links to resources on spiders.
Search Engine Robots and Other User Agents
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.