Posts 2 Posts has long been the “go to” plugin for what's called “many to many” relationships in WordPress. Silviu-Cristian Burcă, who goes by Scribu, has maintained the plugin for a long time. The plugin has over 20,000 active installs on WordPress.org.
Many to many relationships is different from WordPress taxonomy system because of how you can connect disparate items in a database. I use it on Post Status, so I can connect arbitrary profiles to organizations, posts, notes, and other custom post types. It has a pretty powerful API and extensibility with
WP_Query, making complex data relationships significantly simpler in WordPress.
20,000 installs isn't that many for user-facing plugins, but considering this is a very developer centric plugin, it's influential on big and complicated websites. I'm honestly not sure of what the best alternatives are.
It's been a while now that Scribu hasn't done major feature support, but he's still maintained it; in a recent commit he changed that policy and is publicly advocated for folks to fork it, but he won't be updating it himself. Hopefully someone will consider adoption to help Posts 2 Posts continue on. I don't really understand why a fork would be preferable to adoption by another party, but I guess either is better than nothing.
If you use Posts 2 Posts personally or for clients (and I expect many in my audience do), you'll want to follow along with the conversation about its potential future, and if you're the right person to adopt it, I hope you'll consider it!