Google Custom Search Engine allows any user to create their own customised google search engine. It's very easy to use and in minutes you can produce your own version of Google that returns results according to your own preferences.
To test the service I've quickly created an unoffical UK National Museums and Galleries version of Google. As its name suggests this search engine only returns results from the websites of national museums and galleries in the UK.
Try searching for 'current exhibitions' or 'job vacancies' and you'll see how useful these customised search engines can be.
There is loads of potential with these. Art in Liverpool could have their own search engine that only indexed results from art organisations, artists, museums, funding bodies and galleries in Merseyside. The Chamber of Commerce could customise to only index results from their members' websites.
Google custom search engines can be added to your own site, modified to fit your site's design and you can make money from them by carrying adsense adverts with the results.