Apache Geronimo is the latest contender in the J2EE 1.4 certified open source application servers.  It falls under one of the most business friendly licenses available, allowing your organization to do more with the application server.  Geronimo is built by the community within the Apache Software Foundation, which has a history for top quality software such as Tomcat, that is used by millions of companies each day.

Apache Geronimo is a best-in-breed application server.  Instead of writing all the components of J2EE from scratch, the Geronimo developers looked into the open source community and saw that there were many great open source projects that companies use every day.  These are top-notch projects such as Tomcat, ActiveMQ, OpenEJB, AXIS, Spring and many others.  Geronimo uses these open source components and pieces them together to offer a full J2EE stack.  Using these open source projects provides confidence in knowing Geronimo is the best of open source.  The following is just a small sampling of the projects that Geronimo leverages:

Component	Open Source Project
Connector	TranQL
LDAP/Directory	Apache Directory
EJB 3.0, 2.1	OpenEJB
JPA	OpenJPA or Cayenne
Enterprise Service Bus (ESB)	ServiceMix
JMS	ActiveMQ
JMX	MX4J
Web Container	Tomcat or Jetty
Webservices	CXF or AXIS

Geronimo is much more than an application server.  It is a kernel based architecture that allows for pluggable components.  This means that you can put together your own application server that allows you to make it as heavy or light as you want.  Very few people utilize all the components of a J2EE server and many application servers use a lot of resources to run an entire J2EE stack.  With Geronimo, you can include only those components that you need, and build the right-sized server for you.  Geronimo currently comes in 2 flavors, the full J2EE stack, and "Little-G", which is just the Geronimo kernel running only a web container.