Google Custom Search

The classic on-site search tool, Google allows you to set up a free custom search box, with limited customisation potential, for including on your site. This can be monetized by Google Adsense, or you can pay $100 annually to remove the ads and open up a wider range of custom options for your search functions. Users recognise the Google brand and feel, so this really is as good as any in terms of an on-site search solution. Premium mode excepted, it’s also forever free – just sign in with your Google account to start generating your custom search code.

http://www.google.com/insights/customsearch/home