So I just ran the source for the first time today and excited to start using it. Hopefully I can get my friends involved.
What is BitMessage?
Bitmessage is a P2P communications protocol used to send encrypted messages to another person or to many subscribers. It is decentralized and trust-less, meaning that you need-not inherently trust any entities like root certificate authorities.
I recommend reading the PDF of the white paper that was written up in 2012 by Jonathan Warren.