Mac: We're fans of Write, the note taking app for iPhone that's full of features but retains a simple interface, and now that app is on its way to the Mac. Currently, you can check it out for free while it's in beta.
Initially, Write had a very simple, menu bar plugin for Mac that operated like a notepad. It's now more of a full-featured writing app, and while it's not quite as feature-packed as its iOS sibling, it's on its way. That said, Write has everything you need from the start for plain text and Markdown writing and note taking, including some syntax highlighting for Markdown, sync support, and a nice set of export options. If you're a fan of the iOS app, it's worth giving this beta of the Mac app a download.