Google has just launched Custom Search Engine. All you have to do is give it a name and select which sites you want it to search from. You have the option of either searching only these sites or emphasizing the results from these sites while searching the web.
You can also embed the search engine on your website via the AdSense for Search program and make some money via the traffic that it generates.
You also have the option of letting users volunteer by submitting their list of sites to be added in the search results.
This helps if you are creating a topic specific search engine, e.g. a search engine that gathers all information about WordPress. It can help if you get visitors to suggest sites to be searched.
I’ve gone ahead and created one search engine which searches through my network of websites.