Geronimo is a Open Source (Apache License) full blown application server. On the homepage it is possible to download two versions – the full Java EE stack appserver and a smaller assembly only with either tomcat or jetty.
Now there is another assembly available at http://gaswerk.sourceforge.net – a “minimal you just get jms” assemlby. This is specially interesting for those wanting to move out the JMS server to another machine, or want to create a JMS cluster.
But, the most interesting assembly surely is the spring deployment server. This Geronimy assembly allows you to deploy “spring applications” – packaged as jars – providing a special bootstrap configuration in the META-INF folder in the jar.