nvNotes[1] was great, but the latest version removed Dropbox syncing (because the developer was using an older API that is being deprecated) and now it's... no so useful. Combined with nvALT[2] on the Mac, this used to be a great combo.

[1] https://itunes.apple.com/us/app/nvnotes-note-taking-writing-...

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