The Pomodoro Technique(TM) is essentially the same, but waltzing in 25/5 time. A longer interval makes more sense to me too, though. Maybe 45/15 or 50/10; either way, it helps block out distractions - "No, self, I committed to working on this for another fourteen minutes."
I know somebody else mentioned it below somewhere, but since this is the top-most entry regarding software, there is another very good multi-platform program called workrave. I used it while at work with the previously mentioned 48/12 division, but 30/30 sounds great, too. I am going to give it a try right now...
I spent ages looking at various timer software and finally found this one - been using it for years:
The best thing is how few clicks it takes to start/reset a timer.