I guess it's not so hard to do this by hand, but I was curious if there was easy ways to include concepts like "Ratable" or "Favouritable" and so on. It's really easy to do by hand I guess, but there are tag libraries for this sort of thing that also provide scopes, tag clouds and the correct migrations with the right indexing strategies... so I thought maybe there might be a similar thing that is all encompassing since these things are basically the same idea.
Thumbs_up and Ajaxful rating are good solutions of this problem. Successfully used in my applications.