<p>We are looking for a open source J2EE Application server for log budget deployments. We are considering JBoss and Glassfish. Which is the best open source application server? Any comparative study available?</p>

A comparison table between JBoss, Geronimo and Tomcat for latter 2007.
This article
has many interesting points to aid your decision and is worth reading:
Geronimo 2 is the clear choice if your
  Java application needs are
  particularly extensive or if you just
  want to leverage total Java EE 5
  compliance.
Although JBoss 4.2 is not
  completely compatible with Sun's Java
  EE 5 standard, the JBoss 4.2 team is
  responsible for many of the
  cutting-edge technologies used by all
  of the servers and added to the
  standard Java EE 5 capabilities.
  Tomcat 6 by itself is a lightweight
  solution. It does not come with all
  the Java EE features and additional
  packages found in JBoss and Geronimo,
  but also doesn't require much memory
  and runs fast even on smaller servers.
In conclusion the author says:
Bottom line: JBoss, Geronimo, and
  Tomcat are reliable, fast, and
  definitely able to handle the traffic
  on large-scale enterprise
  applications. Furthermore, many of the
  high-demand Java server technologies
  presented in the features comparison
  originate from the open source world,
  not the commercial one.