ergophobe - 5:29 pm on Jan 25, 2012 (gmt 0)
I used to use Popularity Contest or Democracy plugins. I'm not sure if either can segregate by category and I'm not certain they're still being updated.
I've taken to doing "most popular" lists manually using my analytics data. Why in the world would a manualate (new word of the day) a process that can be automated? Because I can subtly work in content that I think should be more popular that isn't.
"Most popular" plugins tend to be self-reinforcing - once listed an item becomes ipso facto even more popular and is hard to knock off its pedestal.