Today, Google has unveiled a new Google Sitemaps program allowing webmasters and site owners to feed it pages they'd like to have included in Google's web index. Participation is free. Inclusion isn't guaranteed, but Google's hoping the new system will help it better gather pages than traditional crawling alone allows. Feeds also let site owners indicate how often pages change or should be revisited. Below, a Q&A on the new program with Shiva Shivakumar, engineering director and the technical lead for Google Sitemaps.
This helps the Google crawlers to be notified of what pages are present and which have changed recently, and to crawl the site accordingly. Google is releasing the Sitemap project under the Attribution/Share Alike Creative Commons license so that other search engines can do a better job as well. Eventually this will be supported natively in webservers (e.g. Apache, Lotus Notes, IIS).
There is also a Sitemap generator which automatically generates the XML file which contains the details about the URLs. But you would require Python2.2 to get the script runing on the server.
Read more about Google Sitemap and also read the Sitemap FAQ