Java RPC: An Overview
Abstract: A collection of links to frameworks allowing for Remote Procedure Calls between Java applications. I ultimately went with the Restlet framework due to its simplicity and integration with GWT.
Restlets: Restlet is a framework for Java allowing to create restful web services. This example shows that it works well both for Java SE and GWT clients. (Java World article from 2008) (Blog post showing Restlet performance is comparable to performance of servlets)
XML-RPC and XML-RPC2: Apache libraries for RPC calls using XML messages. XML RPCs evolved into WSDL. Provides client and server.
Using WS-Security with Axis2: Article showing step by step how to configure Axis 2 to use authentication.
Axis2 module rampart: This module can be added to Axis2 in order to support encrypted messages.
Spring Remote Framework: Allows to use a number of remoting technologies (RMI, Hessian, Jax RPC, ...) - needs to be configured using Spring Bean XML files.