- IntoDNS (www.intodns.com)
Completely free and really sophisticated information about you domain name.
- DNS Check (http://dnscheck.iis.se/)
Also very good tool but not as sophisticated as IntoDNS
- How is my DNS? (http://www.howismydns.com/tools.php)
Not my preferred tool but I always use it to cross-check results with previous two tools.
- DNS stuff (http://www.dnsstuff.com/)