Microsoft, Google, Yahoo! Unite to Support Sitemaps
http://www.sitemaps.org/
"Sitemaps" is a simple way of describing the content of your website. It was introduced by Google last summer and has really taken off.
The feature I think really makes all the work behind it worth while is that you can tell the crawler exactly which URLs have changed (or are new), letting them crawl those first, instead of having to recrawl the whole site to find them. Great stuff!
Even if Sitemaps did not provide any additional value, the value you gain by having to crawl your own website is worth a lot. For most people, it's the first time they act like a search engine
John
