Simon Hayes is an awesome software engineer and writer. We won't attempt to lie some of our software has been inspired by his software, including yWriter for instance. We have worked with his software in the past, loved it and only have created something like it of our own because it didn't quite do what we wanted, or perhaps the way we wanted to do it. That said his software is great, works perfectly and is well worth checking out. Most is actually donation-ware, or freeware. He has one commercial app at the time of writing this.
Double mention here because this is the writing software we used for many years until deciding we wanted to work slightly differently, and gaining the ability to make what we wanted. It uses scene based project management split while our software in production just uses chapters. If you prefer to work scene by scene, use yWriter.