Back in October 2008, GlassFish introduced a feature designed to greatly improve the development experience by preserving the HTTP state across redeployments. That feature coupled with IDE incremental compilation and smart deploy-on-change in appserver plugins has been a big time saver for GlassFish users.
The feature is now known a "Active Redeploy" to reflect that Stateful Session EJB instances and persistently created EJB timers (in addition to HTTPSession instances) are now all supported in GlassFish 3.1. Shing Wai discusses the changes, including a new 3.1 redeployment option when using the
asadmin command line and its deployment descriptor equivalents in both
To see the feature in action you can check out these (somewhat older) screencasts : #1, #2. As you'll see in those examples, this feature is tightly integrated with the development tools, both NetBeans and Eclipse, to provide a save/reload development paradigm. The reference documentation detailing this is here.