Jump rope. Refer to the Coach Davies Renegade Rope Training article at TMAG.COM for specifics.
I have been doing this for a few weeks now, have dropped bodyfat and gotten in very good shape cardiovascular-wise whilst gaining a couple of pounds of bodyweight.
I do 3 minutes of rope jumping (~90-120 turns per minute) and rest for 45 seconds (I don't do anything in between "rounds" of jumping like the above mentioned article suggests). I repeat this for 5-8 cycles. For the first two weeks I thought I would die about half-way through the workouts, but eventually it became easy and I NEVER EVER EVER get winded when lifting weights now, as I used to have to sit down between sets.
I do this workout 3-5 times per week usually on off days or a few hours after my weight workout.
Start off this regimen with 3 two minute rounds with one minute rest periods at first and add an additional 3 minute round per session until you reach six or so cycles per workout. That is my recommendation. Sprinting is also good. There is a lot of bullshit surrounding cardio and muscle loss. Personally, I have always been more muscular, stronger, and bigger when implementing cardiovascular exercise into my training regimine. However, I do believe it somewhat limits MAXIMAL (one-rep max) strength so I wouldn't advise powerlifters to incorporate this type of exercise into their training. But my repping power seems to increase very well, that is what I meant by cardio making one "stronger".
I am drunk as I type this, just to let all of you know.
"Beer...a temporary solution to a permanent problem." - Homer Simpson