I can recommend NValt, a fork of Notational Velocity:
http://brettterpstra.com/projects/nvalt/
it doesn't work in the menubar, but you can configure a shortcut to show/hide the NV window if you want. It's also got a builtin search kind of like spotlight, which you can use to open notes quickly. Screenshot:
per default it uses a database, but you can also save the notes in a specified directory as plaintext, RTF or HTML files. Markdown support is not first class, but it works when you save as RTF or HTML.