Another alternative, admittedly not open-source, is Recollectr (disclaimer: built by me.)
Recollectr was inspired by prior projects like Notational Velocity but aims to be a lot more - omnibox, markdown support, reminders; and for paid users: revisions, note-linking, and sync. I built it because I felt like other note-taking apps just weren't fast enough and they broke my concentration.
It's quite late here but I'd be happy to answer any questions tomorrow!