These free tools have been designed to validate and check the health of your public DNS servers.
Free Secondary DNS for your domains. If your looking for another DNS to put your domain in, this is for you. Providing additional secondary servers can greatly improve the reliability and accessibility of your domain. Should your primary DNS become unavailable due to network problems, your secondary DNS entries come into play, the more DNS servers you have, the greater the resilience to errors.
On the topic, check out BuddyNS as well. Unlimited domains for free, fastest synchronization with primary, 4 geographically sparse servers, IPv6 ready.