"There finally exists a kind of wiki-like linking in Notational Velocity, in the ability to turn any text into a link by placing it inside [[double square brackets]]. If that text is the title of an existing note, then clicking it will jump to that note, searching for it in the process. (The Deselect button in the search/title field will double as a back button.) If no such note exists, just press return to create one with that title, exactly as you're used to doing already." (See here.)
ResophNotes (Windows) had this for a while.