nvALT - A fork of the original Notational Velocity with some additional features and interface modifications
Evernote - Bring your life's work together in one digital workspace. Evernote is the place to collect inspirational ideas, write meaningful words, and move your important projects forward.
Notational Velocity - Notational Velocity: modeless, mouseless Mac OS X note-taking application
Joplin - Joplin is a free, open source note taking and to-do application, which can handle a large number of notes organised into notebooks. The notes are searchable, tagged and modified either from the applications directly or from your own text editor.
Zim Wiki - Zim is a graphical text editor used to maintain a collection of wiki pages. Each page can contain links to other pages, simple formatting and images.
Google Keep - Capture notes, share them with others, and access them from your computer, phone or tablet. Free with a Google account.