Niraj Chauhan

This plugin will help the user to add a drop-down-custom-taxonomy box with the help of small code on his desired location.
<h4>Options</h4>
just place this small code into your theme file and the drop-down box will appear on that page 
code: 
for example, if you want to display it on home page, 
then navigate to index.php, the inside that put this code, if you want to display the custom taxonomies, then just replace category with your custom taxonomy name.

Drop-down-taxonomy

This plugin will help the user to add a drop-down-custom-taxonomy box with the help of small code on his desired location.

Damian Gostomski

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:
<pre><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',
 ),
));
</code></pre>
Read more about <a href="https://codex.wordpress.org/Function_Reference/register_taxonomy" rel="nofollow ugc">registering taxonomies in the codex</a> 
This is heavily inspired by previous work by <a href="http://justintadlock.com/archives/2011/10/20/custom-user-taxonomies-in-wordpress" rel="nofollow ugc">Justin Tadlock</a>

User Taxonomies

Simplify the process of adding support for custom taxonomies for Users. Just use register_taxonomy and everything else is taken care of.

Drop-down-taxonomy vs User Taxonomies