With the release of version 4.0 of Stream Deck software, we released the Stream Deck SDK. This allows the community to create custom functionality and integrations for Stream Deck.
Stream Deck 6.4 or later
Plugins can be found and installed from Elgato Marketplace.
Stream Deck 6.3 or earlier
Plugin can be found in the Stream Deck store which is accessed by clicking on the store icon at the top right of your Stream Deck software.
Stream Deck SDK
To begin making a plugin, take a look at the Stream Deck SDK documentation.