Bitmessage is a P2P communications protocol used to send encrypted messages to another person or to many subscribers. It is decentralized and trustless, meaning that you need-not inherently trust any entities like root certificate authorities. It uses strong authentication which means that the sender of a message cannot be spoofed, and it aims to hide "non-content" data, like the sender and receiver of messages, from passive eavesdroppers like those running warrantless wiretapping programs. If Bitmessage is completely new to you, you may wish to start by reading the whitepaper.
Here is a simple echo server which will send your message back to you after being received: BM-orkCbppXWSqPpAxnz6jnfTZ2djb5pJKDb
Bitmessage should run on any OS though it is only lightly tested on OSX. The start-on-boot and minimize-to-tray features are only implemented for Windows thus far.
Security audit needed
Bitmessage is in need of an independent audit to verify its security. If you are a researcher capable of reviewing the source code, please email the lead developer. You will be helping to create a great privacy option for people everywhere!