I've added an addon exchange for both Construct 2 and Construct 3:
Construct 2 Addon Exchange
Construct 3 Addon Exchange
The addon exchange is hosted on Construct.net as previously explained because our eventual goal is to shut down Scirra.com and make Construct.net the home of everything Construct 2 and Construct 3.
There's a few more features to add - but it's basically complete. We hope in the future to add some sort of API so you can manage/update your addons from the editor directly, but that might take some time to complete!
Please do NOT upload addons to these exchanges unless you are the copyright holder otherwise your account may be suspended.
At some point in the future, this will support paid addons. But for now, it's open source only.