There are any number of ways: HTTP (which might be easiest), shared files, web service, JMS, Email, shared DB, TCP/IP, ...
Direct access to objects or sessions in another web app can be enabled on some servlet engines, but is server-specific. For
Tomcat, have a look at the
crossContext attribute of the Context. It enables the use of RequestDispatchers across web apps.
[ August 24, 2006: Message edited by: Ulf Dittmer ]