Where can I find extensions and services for my Mac

I love the ability to add features on to Safari or Chrome or Firefox.

On the Mac under System Settings, I keep looking at extensions and services. I'm sure that there must be some that would be just as helpful.

Is there an official list where I can find these and be relatively sure that they are not going to turn my Mac into a toad. ;) Are there ratings and reviews? I somehow feel that some have developed solutions to many of the things that we all face.