Favourite interval training methods?

[J.C.]

What are your favourite methods of interval training for cardio and conditioning?

I really like this pyramid drill on the rowing machine. It only takes about ten minutes but its intense enough to really build some power.

30secs hard/30secs light
60secs hard/60secs light
90 hard/90 light
60/60
30/30

So you work up the pyramid then back down. 30 seconds is short enough to be all out, 90 seconds is a test of mental determination but still short enough to be sprinted, and then you work down. If you have enough energy it can be done again after a few minutes light.

How do you like to do intervals and what purpose do they serve?

[VikingWarlord]

Of all the stuff I've done, I like 30:30 kettlebell swings the most. I've done 8 Seconds of Doom, Tabata, and plenty of other more "normal" interval schemes using a treadmill, elliptical, recumbent/upright bike, rower, and jump rope but the swings tear my ass up harder than anything else for general conditioning.

I just did 12 minutes of 30:30 swings with a 16kg bell yesterday and, when I was done, my heartrate hit 204bpm. It was glorious.