It's a free service to store notes and sync them across different clients. I use it through some third-party apps since some years and I find it very efficient and lean.
On their website they say that they are relying on the Simperium libraries ( http://simplenote.com/developers/ )
On maemo there's a client that still works very well (SnXM), being able to use it also on my N9 would be fantastic.