2012 update: Mono is no longer recommended for running yWriter. Instead, you need to install Wine and then use a few simple commands to set up your system.
Download yWriter5 here, and follow the instructions for running Wine/yWriter5 on the same page.
Below is the older tutorial for getting yWriter running under Mono. (No longer the recommended option.)
yWriter5/Mono on OS X
Tutorial by Greg Kennedy, July 2010
yWriter5 is a handy word processor designed for people writing novels and lengthy documents. It helps organize your thoughts and has cool features to reduce the chaos of writing. It's also free. Compare to Scrivener, zWrite, etc.
Though there is no OS X installer or guide on the site itself, you can get yWriter5 working on OS X - using the free Mono library, and the Linux version of the software. Here's how.
Mono is a platform-independent language that lets people write GUI-driven applications and run them on multiple operating systems without needing to recompile. Lucky for us, yWriter5 is a Mono app (written in Visual Basic 2008), so if we get a Mono library for Mac, we can run yWriter5. Browse over to mono-project.com and click on the Download link.