iA Writer - Minimal Design, Maximum Focus
StoryMill - Developers and publishers of Mac, Win, and iOS productivity, home and office and writing software such as MacGourmet, Paperless and many more. Based in MN.
MacDown - MacDown is an open source Markdown editor for OS X, released under the MIT License.
Manuskript - Open-source tool for writers.
MWeb - Pro Markdown writing, note taking and static blog generator
Novelize - Novelize is an online novel writing app designed to keep authors on track.