Apache Geronimo v2.2 is a Java EE 5 Certified server. Installation packages are available for both Windows and Unix (including Linux, Mac OS X) operating systems. In addition to the platform specific downloads, Geronimo is available in three main distributions which are the Java EE 5 Certified (Big-G), Little-G and Geronimo Framework. As suggested by the name, only the first one has full support of all the features and functions in the Java EE 5 specification and has passed SUN's Java EE 5 Certification Test Suite (CTS).
Little-G distribution includes a Web container and a subset of other modules from the certified Geronimo distribution. This is not a Java EE 5 compliant server but it is probably more appealing to users who look for a smaller footprint with specific functionality. Java EE 5 and Little-G assemblies are provided with Jetty or Tomcat Web Containers. You can find more information about the Jetty project here. More information on the Tomcat project is available here. Both are excellent Web Containers that are compliant with the Servlet 2.5 and JSP 2.1 specifications. Your choice may be driven by your past experience and particular functionality requirements.
The third distribution, Geronimo Framework, offers just that, a stripped down framework for you to build up your own custom Geronimo server based on the available plugins. Unlike Little-G, this distribution does not include a web container, it only provides the bare minimum support for installing plugins. This is particularly practical since Geronimo v2.2 is built up entirely from plugins, see the Administering plugins section for additional details.
You can find the source code and binary files of Geronimo v2.2 at http://geronimo.apache.org/downloads.html.
You can also build a server from source code using either maven or eclipse. Look into Building Geronimo from source for the details.