MODX Revolution makes add-on installation simple. The Installer (aka Package Manager) can be used to install add-ons from the default MODX provider, from packages you upload directly to your server, or from third-party providers.
To install a package locally, click on the arrow on the right of the button. This will scan your installation's core/packages/ directory and display a list of any packages that are not already installed. Just upload the package.zip file to your core/packages/ directory and you can install it from there. However, these add-ons will never have an option to upgrade them.
There is also the option of uploading a package, which will let you select the package from your computer, upload it and add it to the list of Extras ready for installation.
As of this writing, I know of a few other providers, ModMore, MODSTORE.pro. and extras.io. Let me know if you are aware of others so I can add them here.
The people at extras.io also have a free repository service for developers who want to have a private provider for their client projects. The modstore.pro repository is also open to developers to provide free or premium add-ons.