The latest in the near-constant stream of beta products from Google hit the web yesterday, Google Custom Search.  Custom Search allows users to essentially create their own customized search index, including only site you choose, and defining the primary keywords to help define the search parameters.  This customized index can then be accessed directly via your own webpage by placing a Google search box somewhere on your site.  You can even have the results output to another page on your site, so users never have to go out to Google, helping you retain visitors as long as possible.  The look and feel of the search box and results page can even be modified to fit with the look and feel of your site, allowing for better integration than the old Google search box.

The trade-off?  Google gets to place ads on your site with their AdWords service.  The good news to that is you still get to share in some of the revenue made from ad clicks and impressions.

Like everything else from Google, the service is free and currently in beta testing.  There's a lot of coverage on this new service, most notably on the Google Blog itself.