Simplenote

Simplenote is a plain text note taking application. It has a history function so you can roll back notes to previous versions, collaboration tools to share and edit notes with others, the ability to publish a note to a URL, as well as tags for organization. The service has a variety of apps for all major platforms and all apps sync effortlessly. Simplenote is also open source, so you can take a look under the hood or contribute to future changes.