Found blog post - Radioactivity 2: basics

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.

I imagine this is of interest here: http://mearra.com/blogs/teemu-merikoski/radioactivity-2-basics