If you're having trouble with focusing at work, maybe an app that blocks distractions isn't quite what you need. Maybe you just need a good, old-fashioned shaming. This AutoHotkey script can give you the nudge you need. This script was written by Neil Cicierega, internet artist and creator of Potter Puppet Pals. When you first fire it up, you can set up to three designated "work" applications. As long as you're in those programs, the little window will show a nice calm blue colour, with a counter indicating how much time you've spent working. As soon as you step out of that window (or if you stop working for too long) the window will turn red and it will remind you to "GET BACK TO WORK". It's an incredibly simple and low-footprint script. If you'd like to check out what the script does before you run it, you can find the uncompiled version here.
Neil Cicierega [Tumblr]