When marketplace developers need to store data related to their apps, they can do this via Active Objects tables (database tables that are prefixed with AO_ then a 6 character hex code which seems to be unique for that app, then another underscore and the table name itself (set by the developer).
When reviewing your environments app usage (could be to get an indication of how used an app is as part of renewal discussions, maybe you're investigating it as part of a performance discussion, maybe your DBA has concerns about table size, maybe you just want to know what's happening in the environment.
For any apps that are still installed, there's a "Plugin Data Storage" page that's available to sys admins that will give you a breakdown of the vendor, the AO_ tables and a count of how many rows in each of those tables.
Below links are for Server/DC
NB: Cannot find equivalents for Crowd or Bamboo
What you may see on this page though is a list of AO_ tables at the top marked either <unknown> or just blank - these are AO tables that were created by an app, but that app is no longer installed, and it seems like the metadata that maps the AO to the app is removed with it. The AO tables are left as uninstalling an app does not remove the underlying data. This is great if you're uninstalling to resolve an issue, or accidentally removing, but long term, you may want to clean up the AO tables for apps you'll never use again.
Unfortunately, I'm yes to find a good/reliable way to find the AO to app mapping - you can Google/search community.atlassian.com for the AO_ABCDEF style table name and you may get a hit, but, it's far from guaranteed.
What I'd like to start with this thread is a community sourced reference page for the AO_ tables and their apps that will be maintained over time (or someone from Atlassian or the vendor community can chime in with a better location).
I'll add a reply for each application with a summary of the tables and apps that I'm aware of (and a list of the AO tables our environment has an unknown as well) and if anyone is able to provide other mappings, then please reply to the applications thread with the AO prefix and app name and I'll monitor and edit the summary table moving forward.