http://goo.gl/0rpEK - Amazing iPad - 34/30 and iPad application that is extremely useful for productivity. Using this application would be very useful in my classroom to time your lesson plan. The designers or the developers of this application have created a very useful and well-designed application.
The idea behind this is that you give yourself a timed period of focus work. At the end of this period you are rewarded with abreak of however long you choose before starting the next session of work.
Or you could use this application to aportion time available to get a job done. You allocate periods of time to various parts of the job and let the timer tell you when you should be moving onto the next section. You get very visual feedback from the application to tell you where you are timewise and you also get the timer to tell you exactly what time you will be finished the set of tasks.