Unofficial Habitica desktop app
Built for OS X, created with Electron. I found that having to open a fully-functioning browser to check off tasks on a productivity app to be a dangerous thing, so I broke it out into its own isolated Electron browser. Plus, nothing beats ⌘+spaceing to something.
Download the release, and move Habitica.app into your Applications directory.
Alternatively, you can build it yourself by getting the source, running npm install && npm run build, and opening the app found inside the dist folder.