Workrave is a good programme but I found it far too intrusive (in fairness it was aimed at people who already had problems so it
forced them to take a break) - but maybe it's evolved since I tried it a few years ago (yes,
I see it has - I will try it again)
Instant Boss I used for years, it is a great app, but I did have a problem with it that if I hit spacebar, or return, when it's "break" window shows - it gets dismissed in a manner that it doesnt show again (IIRC it went to something like 'extend the break'). Hours later I'd be (probably surfing and) wondering what happened. This was a big problem for me cause I work with a drawing programme where I'm constantly hitting return
and the spacebar... (also a big problem cause if I didnt get the reminder again, I'd just end up surfing
instead of working)