Simplenote uses Simperium to store and sync notes across multiple app platforms. The Simperium libraries handle all the networking so developers can focus on building a great user experience.
Contribute to Simplenote
All of the Simplenote apps are open source and ready for your contributions! You can find the source on GitHub:
Electron (Windows and Linux): https://github.com/automattic/simplenote-electron
The README files on each of those repositories have all of the instructions you need to get started!
Third Party Applications
Legacy Simplenote API
Some older applications may still be using the legacy Simplenote API at https://simple-note.appspot.com. Note that this API has been deprecated and will no longer function after October 1, 2018.