A short list of cool DNS Tools sites. This list is definitely not complete, but it is a good start… 🙂
http://network-tools.com/ – nice DNS and network-related tools
http://www.mxtoolbox.com/ – very clean and effective tool
http://www.dnsqueries.com/en/ – offers a nice thorough report
http://iptools.com/ – variety of one-off tools
http://www.intodns.com/ – another nice through report offered here
http://dnssy.com/index.php – once again… report. 🙂
http://www.openspf.org/ – cool site if you want to learn more about SPF. Also has a nice wizard that guides you through the syntax of your SPF record
http://www.kitterman.com/spf/validate.html – SPF record validation tools, both live and staged.