Try nvAlt. It saves all notes to .txt or .rtf, supports markdown (limited), and is super fast. I just switched to it my self and love it. I only wish I could attach files to notes. OSX only but there is a cross platform clone, the name escapes me though.
The Notational Velocity platform is pretty sweet! You might be thinking of nvpy - https://github.com/cpbotha/nvpy . I haven't tried it. Instead, I use nvAlt on Mac, Simplenote on my iPhone, and ResophNotes on Windows.
