Gymboss. Sounds like the steroid-popping walking anatomy chart who seems to live amongst the free weights at the local fitness center. Nope, just a handy little timer thingie that you can use for all kinds of training stuff.
The Gymboss basically lets you program two alternating time intervals, e.g. (that's Latin for "example given", they were really big on abbreviations) a three minute round followed by a one minute rest. The periods can be as short as two seconds and as long as 99 minutes.
The alarm can be set for durations of 1, 2, 5 or 9 seconds, or you can set it to vibrate, useful when you don't want to annoy others. It's fairly tough, no problems to date, and very useful for anything interval related, e.g. (see, there it is again) climbing laps on walls, sprints on your bike, blah-blah-blah. Handy belt clip on back.