Simplenote

Simplenote is a note-taking application with Markdown support. In addition to being accessible via most web browsers, cross-platform apps are available on Android, Linux, Windows, iOS, and macOS.
Simplenote has an externally accessible API, allowing other clients to be written: macOS Dashboard widget DashNote; nvPY, a cross-platform Simplenote client; amongst others. In addition, the macOS program Notational Velocity and the Windows utility ResophNotes can also sync with Simplenote.