There is a list in the wiki
This is a pretty extensive list of the approved addons. However one set of addons missing are the Filter addons.
In Gramps, you can go to the menu Help >> Plugin Manager
You will either open the installed Plugin Manager or the Enhanced Plugin Manager which is itself an addon.
Both will provide a list of all plugins. Some of the plugins come with the Gramps installation and listed as “Builtin” in the enhanced version. The extra addons will be listed as “Installed” or downloaded to your computers as “Available” but not installed (which usually means there was an issue with the addon)…
Addons in the installed Plugin Manager are not separated between Builtin versus Installed. But you can see which by selecting the Info button to check where the addon is located; either in the Gramps install files or in your user plugin directory.
And of course, you can go to that plugin directory to see what is installed.