NinjaRMI is a free, ground-up reimplementation of Java Remote Method Invocation. It supports a number of features not found in Sun's RMI suite, including new communication semantics (one-way, unreliable, and multicast RMIs) and API enhancements (e.g., the ability to determine the hostname of the peer in an RMI connection). And, because all of the source code is provided, you're free to add new features as needed.
