Google’s Sitemap beta.
It is something similar to the ping service provided by most of the blogging softwares.
So if you have a site make sure to add a sitemap to help google crawl all the pages. You can use Sitemap Generator to create a sitemap and submit to google. Read more about it at the sitemap faq.
This particular line from the faq caught my attention:
We don’t guarantee that we’ll crawl or index all of your URLs. However, we use the data in your Sitemap to learn about your site’s structure, which will allow us to improve our crawler schedule and do a better job crawling your site in the future. In most cases, webmasters will benefit from Sitemap submission, and in no case will you be penalized for it.