iA Writer is distraction-free writing app for iPhone, iPad, and Mac. It features an extremely simple UI whose sole intent is to help you focus on your writing. There isn't any fancy formatting options or an overwhelming amount of features. It's just a simple screen with a nice font and a keyboard.
This is exactly what makes it so awesome.
One of the cool features on the iPad (and Mac) is Focus Mode. When turned on, everything except the sentence you are writing fades so that you aren't distracted by something you've already written. For me, I know my eyes tend to wander, so if I notice a missing comma while writing, my thoughts get interrupted until I fix it. Focus Mode removes that distraction -- instead, I'll catch that missing comma when I proofread.
It also includes iCloud and Dropbox integration, so switching between your devices is seamless. Start writing your Mac, continue on your iPhone during your lunch break at work, and finish up on your couch with your iPad.
I've been using iA Writer for a couple months now for writing longer articles and editorials for iMore. It really helps me focus without the distracting UI of WordPress, where I used to do all my writing. Admittedly, I normally use it on my MacBook Air because I like using the physical keyboard; however, there have definitely been times where I've decided to continue working on my article while curled up on the couch with my iPad.
If you do any sort of writing that's not just emails, Facebook messages, and Twitter - I highly recommend iA Writer. You'll be amazed at your increased level of productivity.