In WordPress 1.2 you can add RSS feeds for each category with a simple one line edit. Edit index.php and look for the wp_list_cats() function call. Change that call to wp_list_cats('feed=RSS') and an RSS feed link will be added. Another option that I’ve enabled is optioncount, which adds the number for entries in each category. So now my category function call looks like wp_list_cats('feed=RSS&optioncount=1'). It turns out that wp_list_cats() has a list of options that it supports. I haven’t played with all of them yet so I’m not sure what they all do. Hmmm, I wonder if this sort of thing is documented somewhere?
Update: I should have noted that this was on my WordPress 1.2 Beta install. I have no idea about other versions.