At the time of this writing, there were over 200 plugins tagged users in the WordPress Plugin Directory: http://wordpress.org/plugins/tags/users. We can actually divide those plugins into a handful of groups as the functionality offered is exceptionally wide. For example, there are plugins for:
- Dealing with various author accounts, co-authoring posts, and multi-author sites
- Constructing membership sites around WordPress where members can get access to premium packages of content based on their subscription model
- Building a classic e-commerce store where everybody can make a purchase from the available directory of "things"
- Building an online forum based on WordPress
- Building an e-mail newsletter sent to given site's users directly ...