I've used countdown timers with quite a bit of success. I don't do it too frequently but I think they can be a nice tool to teach pace and even effort distribution. There are a couple ways to handle it:
1. Set out cones at exact distances that you want to monitor pace.
2. Put the athlete's beeper watch at a set interval. With a little creativity you can use this for even when an athlete isn't running nice an easy times like 48s (6s/ 50m). For example, to do this for a 47s 400m runner you just set the beeper time the same but increase the distance of each interval by a meter or two (it will vary depending on the pace). So instead of having 50m intervals monitored you monitor 51 or 52m intervals for pace.
3. You could call out pace as mortac mentioned.
4. You could actually create a countdown timer that was accurate to the 10th of a second quite easily if you have a little electical engineering know-how. All necessary parts could be bought at Radio Shack. I was going to do it last year but the finished product would have been a little too bulky for my likeing.
5. If you have a group of varying abilities, you could set out cones at set intervals (50m seems to work best to keep even distribution / pace) and then have each ability-matched group have a leader with a beeper / countdown watch and tell them to be 'a little ahead' or 'a little behind' at each beep.