This plugin exposes sitemaps for your site. See sitemaps.org for details. It makes it much easier for search engines to spider and index your site.
The sitemaps are dynamic; you don't have to run a cron job or anything.
To install this plugin, add the following to your config.php:
There is an admin panel to configure the plugin. The main configuration options are for adding the sitemap keys from e.g. Google so you can verify with the Google webmaster tools.
The sitemap itself can be found at ../sitemapindex.xml