Entity Systems are a new way of organising your game data.
The basic idea comes from the famous t-machine article about entity systems as a possibility for mmorpg development. Zay-ES is pspeed’s Entity System which he uses for his game Mythruna. Entity Systems aim to make your project more clear and dynamic.
Entity System Introduction
A lot of very up-to-date information on the Zay-ES entity system can be found on the jME3 Forum. There are example projects, example code, and a lot of general help.
Zay-ES can be downloaded via SVN from https://github.com/jMonkeyEngine-Contributions/zay-es
It also now available as a jMonkey SDK plugin.