Google has long offered the ability to add search boxes to your website.  For big projects, they've provided a host of enterprise solutions.  But now they're taking site search a step further with the introduction of the Google Custom Search Engine, or CSE.

More than just a Google search, a CSE can be restricted to certain webpages or just your own.  It can include everything Google indexes.  Or it can just search your site.  The results are displayed inside an IFRAME that Google supplies the code for.

Even better, if you have an AdSense account you can link it to your custom search engine and make money off of all the results.  You can even invite selected partners to help add to your search results with an automatic invitation system built right in to the CSE's control panel.

Google Custom Search Engine is free and seems like a fairly easy and straight-forward endeavor.  If you need to add site search, this looks like an excellent service that could really pay off with a linked AdSense account.