Robust facilities for management and monitoring is one of the most significant ways GlassFish shines as compared to most open source Java EE application servers. Indeed as a former independent consultant, I have personally seen multiple cases where customers chose GlassFish over other options for this very reason. Folks in operations are usually particularly pleasantly surprised to learn that an open source application server could be so friendly to them (although clearly application servers like WebLogic shine even brighter in this area).
In a very well written blog post DZone MVB Dustin Marx describes some of the ways for monitoring and managing GlassFish. He talks about the admin console (of course), asadmin, JMX, the GlassFish REST interface and Ant support. It's a very worthy read if you are evaluating GlassFish or already have it deployed to production (in which case I'd say this is a blog entry worth keeping in your bookmarks or forwarding to the operations folks).