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

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 for WebMasters
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.

Spider Track
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.