My solution to Excercise #2 at http://krondo.com/?p=1333

looping_counters.py

#! /usr/bin/env python

from twisted.internet import reactor
from twisted.internet.task import LoopingCall

class LoopingCountdown(LoopingCall):
    _instances = set([])

    def __init__(self, counter):
        LoopingCountdown._instances.add(self)
        self.counter = counter
        super(LoopingCountdown, self).__init__(self.count)

    def count(self):
        if self.counter == 0:
            LoopingCountdown._instances.remove(self)
            self.stop()
            if not LoopingCountdown._instances:
                reactor.stop()
        else:
            print "{0}...".format(self.counter)
            self.counter -= 1

print 'Start!'
LoopingCountdown(15).start(.5)
LoopingCountdown(5).start(1)
LoopingCountdown(7).start(2)
reactor.run()
print 'Stop!'