Google Custom Search API

Google Custom Search enables you to create a search engine for your website, your blog, or a collection of websites. You can fine-tune the ranking, customize the look and feel of the search results, and invite your friends or trusted users to help you build your custom search engine. You can even make money from your search engine by using your Google AdSense account.

You can create a search engine that searches only the contents of your website, or you can create one that focuses on a particular topic. You can use your expertise about a subject to tell Custom Search which websites to search, prioritize, or ignore. Because you know your users well, you can tailor the search engine to their interests. Your search engine can take into account the context in which your users are searching. For example, when an avid cyclist searches for "wheel" on Google search, she will have to sift through hundreds of results on automobile tires, steering wheels, or Buddhist wheels. This is because Google search cannot discern that the intended context is "wheels for bicycles." A custom search engine for bicycles, on the other hand, would search only pre-selected websites on bicycles and give relevant results to the cyclist.
More information: http://code.google.com/apis/customsearch/v1/overview.html