We've been tracking the growth of Sitemaps on the web. It's been just 2 years since Google, Yahoo and Microsoft co-announced the Sitemaps directive in robots.txt, and it is already supported in many millions of websites including educational and government websites! At the WWW'09 conference in Madrid, Uri Schonfeld presented his summer internship work studying Sitemaps from a coverage and freshness perspective. If you're interested in how some popular websites are using Sitemaps, and how Sitemaps complement "classic" webcrawling, take a look:

At Google, we care deeply about getting increased coverage and freshness of the content we index. We are excited about open standards that help webmasters open up their content automatically to search engines, so users can find relevant content for their searches.