Show words different sizes based on frequency used hack?

I don’t know the name of this add-on so I am having trouble searching for it, but you all know what I mean.

I need something that can display the most used words in my posts and make the words different sizes based on how often the word is used. I have seen this elsewhere on blogs and had a hack for my old MT blog, but now I need one for WP. I am sure this is around, I just can’t put my finger on it…