Daniel Jalkut recently released a new version of MarsEdit which, in my eyes, is hands-down the best MacOS application for writing for the web. I do not use anywhere near all of its features, but nearly everything for this website is written using MarsEdit. I adore it. This is the first new version in years and it is a completely fair $60, but MarsEdit 4 users can get a copy for half the price.
Update: I originally said Brent Simmons was responsible for MarsEdit 5 because I do not know how time works. Sorry or thanks to both developers.