A Google web-based console that provides additional insight into a registered website's search performance and potential issues.
Google established the "Webmaster Tools" console with the purpose of providing more transparency to website owners and web-developers with issues that arose out of being discovered, crawled and indexed by the search engine.
The service is provided as web-based console requiring both a Google account to login, and also to have the website 'registered' or 'verified' and matched to the login account using a number of different methods (Google Analytics, Meta Tag, File-Upload and DNS text entry).
The console is located at https://www.google.com/webmasters/tools/
Further information, including help using the tool, can be found at Google's own help resource: https://www.google.com/support/webmasters/bin/topic.py?hl=en&topic=8465
