DMARC Reporting Sources and Compliance - This is a list of DMARC compliance capability of particular sending sources. Very useful for planning implementation.
LearnDMARC.com - An interactive Email Authentication tool that steps through SPF/DKIM authentication, and DMARC alignment/results.
Proofpoint Email Fraud Defense - Note: If you use Hosted Proofpoint, Essentials, or send to Proofpoint customers, you must use EFD to see data processed by their infrastructure for your domains; Currently reports are not sent externally.