Online Tools

Shodan Scanner

https://www.shodan.io/

Cricket Liu's DNS Advisor

http://ww2.infoblox.com/services/dns_advisor_tool.cfm

Network Tools

http://bgp.he.net/

Email DKIM/SPF Tests

http://www.kitterman.com/spf/validate.html
https://www.mail-tester.com/spf-dkim-check

SPF Generators

http://www.microsoft.com/mscorp/safety/content/technologies/senderid/wizard/default.aspx
http://spfwizard.com/

DKIM Generator

http://dkimcore.org/tools/

Reverse DNS Lookup

http://mxtoolbox.com/ReverseLookup.aspx

DNS Cache Check

http://cachecheck.opendns.com/