Google Custom Search

Transform your website in a specialized search engine with an index composed of the pages on your site or other sites carefully selected, to which applies the algorithm of Google!
This widget is used by major sites like the New York Times and MacWorld.

To mark the anniversary of the three years of the service, Google has made several developments described in the blog of custom research. It provides the following new features:

Specialized search themes.
To choose the appearance of page results in a list of themes.
Structured search custom.
More complex, structured search requires that we provide meta-data to the engine.
Search on Wikipedia.
Replace the wiki engine with that of Google.

How do you use it?

Google provides a JavaScript script to put on your pages to display the custom search box. This script is generic but can be customized online after registering on the Google site.
A form also allows the by entering the URL of your site to test results that provides this search engine.

Create on the fly.

Personalized Search and Adsense

If you have an Adsense account, the custom search box can be combined with your program and display search results on your site that will be supplemented by advertisements.

See the Adsense tutorial.

For business

It is also proposed a paid service whose cost depends on the number of pages and annual queries. This service allows the webmaster to weight searches based on date and also favor certain parts of the site.

See Google Site Search.

Linked Custom Search

The difference with the standard version is that the specification of the search engine is hosted on your site and not on the online service on Google.com/cse where it should be defined.
The research itself is still done by Google, the URL of your site is sent by the script of specialized research.

The advantage is that you can create multiple engines customized for different pages or different users.

See the Google documentation on Linked Custom Search.

Wikipedia

Custom Research should now be incorporated into Wikipedia with the ability for users to replace the search tool from Wikipedia with that of Google.
This enables a contextual search: information is targeted to the content of the current page.

More

Custom Search, the Google service.