Old topic, but in case it's helpful to anyone else who finds this, I always preferred W3C's various tools; after all they are literally the
standards organization for the web. Their validators won't hold your hand but they'll certainly let you know when you've forgotten a bracket.
I've also gotten good use out of Pingdom's tools. Useful analytics (pageload speeds, requests and content info, etc.) along with easy-to-understand suggestions that can lead you to substantial performance improvements.