I often find myself needing to take notes and todos about technical projects I'm working on. Previously, I would have used TextEdit but it's difficult to write for long periods of time and not get distracted by other apps.
The main feature of the app is its Focus Mode that fades out all the text except the current sentence you're working on. This plus the ability to go full screen makes a very focused writing environment.
iA Writer supports built-in Markdown formatting which makes it great for writing technical notes. The built in support for iCloud and auto-saving means you can concentrate on writing and leave everything else up to iA Writer.
iA Writer is available for Mac, iPhone and iPad and is available on the App Store for £5.99 and £0.69 respectively.