Geronimo
Apache Geronimo is a new effort coordinated by the Apache Software Foundation to make a J2EE compatible container.

JBoss
JBoss is advanced middleware with a full J2EE based personality that IT departments look for. But that is not all, the OEM and ISV community embraced JBoss as a highly flexible service oriented architecture on which to build their own products.

JFox
JFox is an Open Source, standards-compliant, J2EE based application server implemented in 100% Pure Java.

JOnAS
JOnAS is the Open Source implementation by ObjectWeb of the J2EETM specification. JOnAS is a pure JavaTM implementation of this specification that relies on the JDK. JOnAS is part of the ObjectWeb Open Source initiative, which was launched in collaboration with several partners including Bull, the France Telecom R&D division and INRIA.

OpenEJB
OpenEJB is an open source, modular, configurable, and extendable EJB Container System and EJB Server. It comes with fast, lightweight EJB Servers for both Local and Remote access. That's right, deploy your EJBs into the container system, then just start the Remote EJB Server from the command line! Or, put OpenEJB in your class path and use it as an embedded library through the Local EJB Server.

Resin
The Resin® high-performance application server features load balancing for increased reliability. Resin encourages separation of content from style with its fast XML and XSL support.