OpenEJB is a simple, lightweight Enterprise JavaBeans (EJB) container and server. You can embed OpenEJB in your own applications, or in a Web application server like Tomcat, or run it as a standalone EJB server. It supports EJB 3.0, 2.1, 2.0, and 1.1, and has partial support for EJB 3.1.
SCOP is a library for writing distributed applications. The functionality includes message-passing, events, and RPCs. It is primarily aimed at C++ programs on Linux, or Java anywhere. Much simpler than CORBA, it is easy to learn, general-purpose, fairly fast, thoroughly documented, complete, and free.
SwitchRMI is a framework for remote method invocation from Java that allows the client to choose from one of several RMI protocols. Objects can be made available over a number of transports; HTTP and SMTP are implemented. Any object vended can respond to an RMI request in any one of the supported protocols. Clients and servers can interact with third party implementations of supported protocols. New protocols and transports can be plugged in to the framework at runtime.