Wikipedia
Simplenote
Simplenote is a note-taking application. In addition to cross-platform apps, it can be accessed via most web browsers.
Simplenote has an externally accessible API, allowing other clients to be written: OS X Dashboard widget DashNote; nvPY, a cross-platform SimpleNote client; amongst others. In addition, the OS X program Notational Velocity, and the Windows utility ResophNotes, can also sync with Simplenote.