As far as I know they are using ScriptCraft which is a Mod for CraftBukkit and CanaryMod which lets you create mods in Javascript. [disclosure: I maintain ScriptCraft] https://github.com/walterhiggins/ScriptCraft/
Hmm, I find $120/year (after official release) quite a high price for what seems to be different open source software and a 4 slot server packed together.