This plugin extends the default taxonomy functionality and extends it to users, while automating all the boilerplate code.
Once activated, you can register user taxonomies using the following code:
register_taxonomy('profession', 'user', array(
'public' =>true,
'labels' =>array(
'name' =>'Professions',
'singular_name' =>'Profession',
'menu_name' =>'Professions',
'search_items' =>'Search Professions',
'popular_items' =>'Popular Professions',
'all_items' =>'All Professions',
'edit_item' =>'Edit Profession',
'update_item' =>'Update Profession',
'add_new_item' =>'Add New Profession',
'new_item_name' =>'New Profession Name',
'separate_items_with_commas'=>'Separate professions with commas',
'add_or_remove_items' =>'Add or remove professions',
'choose_from_most_used' =>'Choose from the most popular professions',
),
'rewrite' =>array(
'with_front' =>true,
'slug' =>'author/profession',
),
'capabilities' => array(
'manage_terms' =>'edit_users',
'edit_terms' =>'edit_users',
'delete_terms' =>'edit_users',
'assign_terms' =>'read',
),
));
Read more about registering taxonomies in the codex
This is heavily inspired by previous work by Justin Tadlock