I’ve had more than a few people ask me since watching the TextExpander/Markdown screencast what was making my Mac only show one application at a time (i.e. hide on switch). I’ve never liked the visual clutter of having different applications layered on top of each other. Therefore, I have sought out utilities to help with this since my Mac OS 7 days. The one I currently use, and have been since 10.2, is LiteSwitch X from Proteron Software. It’s a really excellent replacement for the built in application switcher and it provides tons more features. That said, the single application mode is not something it does out of the box. Therefore, here is a screenshot of my settings:

I hope this helps those of you who have asked and others who want to use it just to bring a bit of that iOS flavor focus to your Mac desktop.
command-h, command-tab kinda guy… but