anything like add_meta_box for categories?

You can add content to the category edit pages by hooking into the edit_category_form_fields (still in the form table) or edit_category_form (outside the table but before the submit button). Similar hooks exist for tags or custom taxonomies, check the source. To handle these extra values you need to find a good hook yourself, make sure you handle it before editedtag redirects you.

There are no clean meta box handlers, you need to write most code yourself. But please let us know how you did it, it might be interesting for others!