If you have adsense, you can use AdSense For Search to let users search either the Web or just your site and earn revenue with these searches. If you don't have AdSense, this will let users search using Google: <form action="http://www.google.com/search"> Search query: <input type="text" name="q" /> <input type="submit" value="Google Search"/> </form> Code (markup): If you want to just let users search your site, you should take a look at htdig.
Another alternative is to check the link in my sig. We've got a search engine that provides free search feeds. So you're not limited to a certain number of searches, and you can completely change the results (i.e. keep the searchers on your site). For example you can pull the search results down from us, modify them, then put them on a page with your own advertising.