When you want to add these extensions to your Tiled installation:
Open Tiled and go to Edit > Preferences > Plugins and click the "Open" button to open the extensions directory.
Download the files in this repository and extract them to that location. The scripts can be placed either directly in the extensions directory or in a subdirectory.
(Alternatively, clone this git repository into the extensions directory)
When using a version older than Tiled 1.3.3, restart Tiled.
(This was necessary because Tiled only watched existing scripts for changes. No restarts are necessary when making changes to existing script files, since it will trigger an automatic reloading of the scripts.)
If you have written an extension that you think makes a great example or would be generally useful for others, please don't hesitate to open a pull request to have it added to this repository or to the list below!