I was having trouble finding something to remind me to take an exercise break every hour or two. A lot of the break timers out there dim the screen and force you to press a button if you want to postpone it. This is problematic for programmers like me - you get into the "flow", and interruptions like that break your concentration, which is Not Good.
It turns out that it's not terribly hard to create your own (unobtrusive) break timer. Here's one that simply shows a Growl notification and makes a sound every hour, on a Mac with growlnotify installed:
growlnotify -m "Take a break"