Addins
Download all addins in a single zip file here: JVC-addins.zip, unzip it to get the jar files.
You install an addin by placing the .jar file in the <mainplanetsdirectory>\addins directory; e.g. as "c:\planets\addins\AutoTax.jar". Next time you start JVC it will autodetect it, and you should go to the game property dialog to activate the addin for your games.
Most addins are open source, and is managed at sourceforge. So if you are a programmer you may want to check out: JVC addins on sourceforge