Sitemap Validator
Validates your (Google) Sitemap against the sitemap standard.
Official links for Sitemap Validator
Sitemap Validator Review
Even though the Google Webmaster tools validate your sitemap, they have very long response times - it takes hours for a new sitemap to be re-validated.
This sitemap validator gives you immediate results, which is great if you edit your sitemap manually or generate it with a tool or CMS.