Sitemap Pinger is an extension that allows you to notify major search engines about your updated sitemap each time new entry is created.
To use it, you'll need to have dynamically biult xml sitemap for your site. You can biuld it using {exp:channel:entries} calls or using dedicated module (NSM Better Meta is capable of that).
The extension supports following search engines: Google, Bing, Ask.com and Yandex. You can also add other search engines (as soon as they support sitemap protocol)
The extension is MSM compatible, so the setings are stored separately for each site. You should provide full sitemap URL, select search engines to ping and choose 'trigger channels'. 'Trigger channels' allow you to send ping not just when any content is submitted, but only if it submitted in selected channels.