Google Sitemaps helps webmasters keeping Google informed about all your new web pages or updates and increases the coverage of your web pages in the Google index.
This project doesn’t just pertain to Google, either: and is released under the Attribution/Share Alike Creative Commons license so that other search engines can do a better job as well.
They offer Sitemap Generator, an open source client in Python to compute sitemaps for a few common use cases.
Google Sitemaps is an experiment in web crawling. By placing a Sitemap-formatted file on your webserver, you enable the Google crawlers to find out what pages are present and which have recently changed, and to crawl your site accordingly.
In 2 simple steps – Generate a Sitemap in the correct format using Sitemap Generator. and update your Sitemap when you make changes to your site.
[via Google blog]