I have been using nvAlt[1] for the past 3-4 years. It is well designed and it is easy to use. All the notes are in text format and can be synced via dropbox (so you can pick up these notes in mobile).

I use Evernote[2] to store notes with images.

[1]: http://brettterpstra.com/projects/nvalt/. [2]: http://evernote.com/