Instead of using apps to focus, I like to have 4 desktop spaces in Mac OS X and assign my apps to one desktop instead of "all desktops."
I jump to a desktop via Mission Control and launch my app. When the app icon shows up in the dock, I can control-click on the app icon to get my Options. I choose "Assign To This Desktop" for my apps.
Desktop 1: Office apps - invoicing app, Digital receipts library, DevonThink, Finder, Pathfinder, etc. All of my business apps go here.
Desktop 2: Productivity apps - OmniFocus, Fantastical, OfficeTime (time tracking app).
Desktop 3: Main Work desktop - I typically go to this desktop when I want get away from my Office apps (Desktop 1) and Productivity apps (Desktop 2). Then I can focus on a big project and stay here. It might be Pixelmator, Adobe InDesign, FIleMaker, BBEdit, Microsoft Word, Microsoft Excel, iWorks, or whatever.
Desktop 4: Entertainment - iTunes resides here because I like playing background music. It might be some instrumental movie soundtrack or my music playlist. The music playlist must be relatively old music (one year or older) that is comfortable enough to be recognizable and becomes background music. I can't concentrate when it's new music I'm trying to listen to. Other entertainment apps that I put here include Twitter which distract from my work. But I typically quit Twitter so that it never dings at me. Usually I refer to Twitter on my iPad and set Twitter to open only in Desktop 3.
I will use Mission Control to go from mode to mode. If I need to go to my office apps, I will hit Control-1 to go to those apps. If I need to go to my productivity apps (OmniFocus, Fantastical), I hit Control-2. if I need to go my entertainment apps, I'll hit Control-4. If I am doing other work, I'll go to Control-3.
To remember my hotkeys, I taped a strip of paper on my desktop monitor's bezel:
(1) Office ( 2) Productivity (3) Main (4) Entertainment
I'm comfortable with using Mac OS X's Spaces to focus. I have been tinkering with another freeware app called Hocus Focus that allows me to hide all background apps when switching between apps.
It allows me to also set a timer for different apps. It will hide a background app after a certain time interval (typically one minute). I can also disable auto-hiding for certain apps (Acrobat or Preview) if needed. This freeware apps is starting to look like something that might just replace Spaces. I'm still experimenting in this area.
Another form of focus for my Mac workflow is to just move my workflow off the Mac. I'll use my social feeds on my iPad. That includes Twitter, Facebook, my Facebook Pages Manager (for the office's Facebook page), e-mail, and whatever app that likes to ding at me.