Modify the source code: This is appropriate for adding core framework functionality and bug fixes. It is not a good choice if all you want to do is add some optional bolt-on functionality and possibly share it with others.

Traditional extensions (Actions, Factories, and Listeners): These types of extensions can still be added. This is a more granular and fragmented option in that large extensions might involve several separate files to deploy (although you could combine them with another ZIP). If all you want to do is add a new listener then this might still be a good option.

Plugins: This is the newest and most comprehensive way of packaging extensions, that is, in a single JAR file. This is probably the best option for most extensions.