Just noticed over at Clusty that they're giving away their Clusty Site Search to schools and non-profits. More from the site:

"Use Clusty to search your website! The forms below can be added to your site's HTML, to provide a web form from your page to cluster the web with Clusty. You can limit searches to a specific tab, the sources you like best, or even just results from your own website."

Related: You can also do a quick site search with Google, or search a specific domain.