Search API’s: Yahoo, Goolge & Bing
The 3 major search engines all offer the possibility to use their search technology in your on-line application. You can offer Google, Bing and/or Yahoo search results to your visitors on your site in your own layout and filters. The most common use is probably of Search APIs is probably website owners offering site search to their visitors. Joomla, WordPress, Drupal and most other CMSs come with build in site search, but for larger websites this produces irrelevant results. The top search companies are just better in sorting by relevance. If you have a relatively large website and relevance in site search is important to you the Yahoo, Goolge or Bing Search API can help you greatly. For all three you need to register for a API key. It is free to get a key, but there are restrictions on the amount of searches you may preform.
These are the URLs you can use to register for an API key.
Documentation can be found here
Google & Microsoft also offers paid services under the names Search Solutions for business & SharePoint 2010 Search.
Next to that Google also offer Custom Search which allows only very minor adjustments in layout, but a lot af filter options. The fact that you can not remove the ads from the SERPs is compensated by the fact that you can make some money from them with your AdSense account.