I was playing around glassfish (JEE 5 compliant open source application server (for those who are not familiar with)). I found it pretty cool. Some of the features I found useful are:
Memory settings and other JVM configuration options are also configurable from admin console (For most of the leading app servers this has to be done in the startup script and the configuration information is spread in two files startup script and configuration xml file). For glassfish all configuration information is stored in domain.xml. Flassfish achieves this by using launcher Java program.
Log levels can be configurable at sub system level (for example you can enable "finest" logging for security sub system to debug some security issues)
Admin console looks pretty neat and clean. No applets (which has heavy dependency on Java plug in and can be big pain)
JSR 196 implimentation. Even though it is part of JEE 5, glassfish impments JSR 196 (Java Authentication Service Provider Interface for Containers). which is a much awaited feature I would say.