Does anyone know a way (either programmatically or through a plugin) to assign tags to users?
I want to build a list of user interests as tags, so I can match related content to their interests.
I think this is what you are looking for: http://codex.wordpress.org/Function_Reference/update_user_meta
Here is a example of how this can be achieved: http://inchoo.net/wordpress/programatically-add-metadata-to-user-accounts-in-wordpress/
It's a quite old question, but I'm answering it as it might help other users who come looking for same.
To assign User Tags, you can use the Plugin. Wordpress has built-in support for it, the Plugin allows you to do it the easy way.
Instead of manually registering User Taxonomies, It allows you to directly create the Taxonomies and provides a template for each tag, You can check the demo link here: User Interests:
You can filter the Title, the listing Or the empty list message as per need using the filters: