You don't need a plugin, it is built-in so long as you are using permalinks. (However looking at the sites in your sig it doesn't look like you are.) You would browse to your category page and then just add "/feed/" to the end of the link.
This isn't for any of the sites in my sig. I have permalinks set to default in the blog in question. What should I change this to in order to syndicate categories?
Browse to your category page, copy the URL and add "/feed/" to the end of it. That becomes the URL to the category feed.