Enter a search query (normally a function or variable name) and you will get a listing of multiple results:
- Link to the page in the Function/Template Tag Reference from the WordPress Codex
- Link to a PHPXref page for your query (at http://xref.yoast.com)
- Link to the phpDoc page at WordPress.org
- Finally, you'll also get a list of resources and pre-populated search links to other Codex documentation and Google web search
Update (2012/10/19): WPLookup is down for the count, link removed.