Wagtail has a large and growing ecosystem of third party apps and packages, but they haven't always been easy to find. We've pointed people at the community-run Awesome-Wagtail list (which is awesome, and which you should read from top to bottom), and to Github topics, and we've highlighted interesting new projects in This Week in Wagtail (you should subscribe!).
The core team has decided to embrace djangopackages.org as the Wagtail package directory, and over the last two months, core team member Coen van der Kamp has been adding and categorising all the packages he's aware of. One result of this hard work is our new page at wagtail.io/packages, which uses the Django Packages API to build a live, categorised listing.
I recommend Coen's excellent article, which gives more background on the current state of Python, Django and Wagtail package listings. Thanks Coen!