Menu bar app for plain text/markdown notes?
I 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 built in search, kind of like spotlight, which you can use to open notes quickly. Here's a screenshot:



By default it saves the files in a database, but you can also save the notes in a specified directory as plaintext, RTF or HTML files. Or you can synchronize it with Simplenote (but Dropbox will work just as well if you use files/db). Markdown support is not first class, but it works when you save as RTF or HTML.