AleHu
Encrypting, anonymous, unlimited size message transfer system
AleHu is an open source encrypted message transfer system that encompasses both server (PHP) and client software (Java).
Give it a shot: You can try out AleHu by simply running the client using the preconfigured AleHu test server!
For further details and instructions on how to quickly install and run AleHu, have a look at the documentation in the Wiki section.