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Seiko makes interval timers:
http://www.amazon.com/Perform-Better.../dp/B00065WZTS
Timex ironman watches work for a lot cheaper if it has countdown-repeat(20 second countdown, and the alarm goes off for 10sec before it repeats) but you may have to count your own intervals.
Finally, the "gymboss":
http://www.gymboss.com/
Almost forgot:
http://www.speedbagforum.com/timer.html
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neato, I'm going to order a GYMBOSS. Totally sweet name.