Here’s a few that I use:
PHP.net search: http://php.net/{query}
WordPress.org plugin search: http://wordpress.org/extend/plugins/search.php?sort=&q={query}
WordPress.org theme search: http://wordpress.org/extend/themes/search.php?sort=&q={query}
QueryPosts.com function lookup: http://queryposts.com/function/{query}