This online magazine reviewed some antiprocrastination tools, including my own Pomodoro Desktop. I have to say that I didn't know this magazine, and I didn't know some of the tools reviewed. But the review inspired me to add some new features to Pomodoro: I liked very much the ideas behind Self Control.
The main idea is to block ports and addresses using ipfw (which is embedded in OSX) during your focus time, with black and white lists. That means you can't possibly look at your email, go to facebook or stuff like in your pomodoro time. It's technically easy to code - just craft and send some commands to ipfw - but very effective.
I'll probably add these features to my Pomodoro in one of the next releases: I can't see a way to integrate smoothly, because Self Control uses its own timer