Aside from dmarcian, here are some other great tools and resources worth knowing about:
DMARC.ORG is where the DMARC technical specification was born. There's a lot of good information to be had on the site, including FAQs, overviews, and news.
Kitterman Technical Services maintains the best DMARC Record Assistant that can quickly create perfect DMARC records for your domain. DMARC's various record options are clearly explained. Use this to avoid typos and to create records that do what you intend them to do!
protodave's DKIM Key Checker can lookup and verify a DKIM TXT record and determine its public key length.
When a bunch of people make new things all based on a new technical specification, interoperability is often rough around the edges. The folks behind dmarcian put together dmarc-qa.com to host a suite of tests that DMARC developers can use to ensure that their code behaves correctly.