I'm a huge fan of the Tabata protocol (20 secs high/max intensity, 10 seconds rest x 8 cycles=4 minutes per round); I do 5 rounds with 60 seconds between each round.
There's an online timer here (with 5 rounds):
Online Tabata Clock - Workout Timer
That site also has downloadable version to put on your ipod. I use a mixing software to overlay the tabata timer over my workout music before putting it on my ipod.
The important thing about doing tabata rounds is that the 20 seconds should be high intensity. If you do 20 seconds of easy work, followed 10 seconds of rest, you will not get the benefits.
This works for both strength exercises or cardio. I've used it with free weights, kettlebells, treadmill, bootcamp-style, cycling/spinning.