Lot's of discussion about other apps but none cover the main selling point for me. FSNotes was created initially as an open source Notational Velocity / NVAlt replacement when NVAlt broke on one of the MacOS version upgrades. It can be configured to replicate the amazing modal search / create interface from those apps.
There is also NVUltra [0] from Brett Terpstra, his successor to NVAlt. It is still in beta after quite a while. From memory it seemed more powerful and a bit slicker overall, but right now I don't need to do anything too complicated and prefer the cleaner native look of FSNotes.
FSNotes has no dropbox integration for the iOS app, but you can still use dropbox as a store and use any other plain text / markdown iOS note app. I use 1Writer.