With Radioactivity 2 nearly out of its beta form, I thought I'd write a bit about what it is and what you can do with it and most importantly walk you through on how to use it. Radioactivity 2 allows you to track any fieldable entities; users, nodes, commerce products, etc. It can be used as a simple view counter but also as a popularity or activity meter, e.g. with it you can create a 'most viewed nodes at this moment' list. There is also limited rules which allows you to create an activity meter for basically whatever you want.