Recs.
Updated
A cross platform note taking app with built in cloud synchronization. Minimal in appearance, this is an app meant for taking notes as simply as possible.
SpecsUpdate
Pros
Pro Very simple and clean interface
As the name implies, Simplenote takes a minimalist approach to its interface. There are no toolbars full of formatting options or extra features like notebooks to group notes.
The entire desktop interface consists solely of a sidebar with your tags and trash filters, the list of existing notes with search, a button to add a new note and a simple view for looking an existing note or writing a new one.
Pro Wide range of great clients with great sync
Official native apps are available on popular platforms such as Android, iPhone, iPad, Kindle, OS X, Windows and Linux. Open-source clients extend this support even more widely to various other platforms, such as webOS and the BlackBerry PlayBook. These apps embody the core philosophy of Simplenote: minimalism and focus on the note taking.
There are multiple unofficial clients (including the excellent Notational Velocity and NValt, a fork of the app with markdown support and other goodies.)