ACAP - Automated Content Access Protocol - Standard being developed on behalf of content publishers to communicate permissions information more extensively than is the case with robots.txt. Project documents, implementation and background information.
Bots vs Browsers - This large database lists user agents in categories and distinguishes between robots and browsers.
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.
Robot IP Address - Brian Dunnintg provides a list of all the major search engine robot IP addresses, by full class C only.
Robotstxt.org - Information on the robots.txt Robots Exclusion Standard and other articles about writing well-behaved Web robots.
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.
User Agent String - Tool from ASAP Consulting s.r.o. for detailed user agent string analysis using an online form. Includes databases of browsers and robots.
User-Agents.org - Large list of search engine spiders, similar web robots, and Web browsers: their web-log identification and links to their originators.
