Below is a is list of URL’s to ping search engines with your XML sitemaps (For more info on Sitemaps see )

Ask.com: http://submissions.ask.com/ping?sitemap=http://www.domain.com/sitemap.xml
Google: http://www.google.com/webmasters/sitemaps/ping?sitemap=http://www.domain.com/sitemap.xml
Yahoo: http://search.yahooapis.com/SiteExplorerService/V1/updateNotification?appid=YahooDemo&url=http://www.domain.com/sitemap.xml
Live Search: http://webmaster.live.com/ping.aspx?siteMap=http://www.domain.com/sitemap.xml
Another Service (Just heard its good to use :S): http://api.moreover.com/ping?u=http://yourdomain.com/yoursitemap.xml

Hope this list helps someone