Easily submit your sitemap.xml to Google, Yahoo, Bing and Ask.
To configure it go into the System->Configuration->Google Sitemap section. You will need to provide a Yahoo AppId if you want to submit your sitemap.xml to Yahoo using their webservice. You can also enable or disable both the extension and the auto-submit feature from this configuration page.
To manually begin submission you can press the Submit button on the edit Sitemap page - which you get to by selecting a sitemap on the Catalog-> Google Sitemap page. Or now the more convenient option kindly provided by Filipe is to simply select submit from the drop down on the grid screen.