I have free php scripts that can ping rss and sitemap to search engines and rss aggregators. Check here: http://www.siteduct.com/web_tools
First of all ensure that your sitemap is upto date, if its just a text change to an existing page then it will be fine but if you have added a new section then you will need to create and submit a new sitemap. Below is a link on how to create a sitemap
I don't know if someone has mentioned that to you but if your site is based on WP, then it is automatically pinged every time you write new content.
I use ping on my Blog, everytime I have a new updates on my blog I started to ping it so that the search engine will know that I have new updates.
Pinging could be helpful specially if you have a new post in your site or blog , it informs the SE that a new page have been published and thus invites to revisit the site or RSS feed or sitemap-file . You can try autopinger.com or ping-o-matic to ping your sites.
I've used Google Ping for years now and it's very quick and effective if you want to ping search engines.