Ask HN: What are the best unique Mac apps?

Mac users: what unique Mac-only apps do you use that couldn't be had on Windows? What apps with unique innovations do you see in Mac apps that you're not seeing on other platforms? I'm doing a bit of research for an article idea...

I'd asked this on Twitter earlier, and got some nice feedback, including Kaleidoscope, BBEdit, Courier, Reeder, MarsEdit, Acorn/Pixelmator, NvAlt, Fantastical, Sparrow, Versions, Adium, and Coda.

Thoughts?