The special searches help section in Google Webmaster Tools was updated for the first time in several years, as of October 2010. Special search results give insight about how your site is indexed by Google.

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Special site searches
This command returns the full list of special search queries:
info:operator
Search indexed pages
View all pages indexed by Google for your site using site:operator
Entering
site:google.com
returns all indexed pages for google.com
Note: Don’t use a space between the operator and the URL!

Google search results for domain Wikipedia.com and eight sub-domains
Search within a single domain or sub-domain
The same syntax is used whether searching an entire domain, or restricting the search to a sub-domain only.
The same syntax is also used to restrict search results to a specific sub-directory.
The command to search only within the webmasters
sub-directory of site google.com
is
site:google.com/webmasters
Exclude pages
To exclude particular pages from search, use a minus sign before the operator.
This would be the command to return results for all indexed pages on the google.com
domain, without any adwords.google.com
pages:
site:google.com -site:adwords.google.com
Pages that link to your site’s front page
To search for sites linking to http://www.google.com
use either:
link:google.com
or
link:www.google.com
Google advises using the first command syntax as it will return more complete results.
Links to pages
Search for all links to specific pages or sub-directories. This command will return all links to the webmasters
sub-directory of domain google.com
link:google.com/webmasters
The current cache of your site
View Google’s archived copy of an indexed web page using the cache:operator
This is sometimes called the cached version of the page. For example,
cache:google.com
displays the most recent version of the Google homepage google.com
as well as the cache creation date. You may also view a plain-text version of the page. This is useful because it shows how Googlebot sees the page.
Pages that are similar to your site
The related:operator
displays websites similar to the site you are looking for. It returns the same results as clicking Similar pages next to a result on the main Google Search results page. Google provides more detail:
This search is like searching a bookstore to find books similar to the first Harry Potter novel. The results could include other children’s books, a biography of J.K. Rowling, or a non-fiction book on children’s literature. In general, use this operator to find resources that overlap. You’ll get the best and most useful results if you use sites that cover a broad range of content.
Google uses several factors to determine the similarity of different sites but does not describe these factors any further, other than stating that
the quality of the sites returned has no impact on your ranking or on how Google indexes your site.