I've been going through the Crossfit site recently and have been really impressed with the work outs. I believe doing this style of training will be the most effective for cardio training but least detrimental to my powerlifting training. Check it out.
Repeat program 3 time through with no rest.
Jog (walk when exhausted) - 400m (down the street and back).
Stair Push Ups - 20 reps.
Deadlifts - 10 reps/120lbs (very light).
Skipping - 40 jumps (not a speed rope).
The whole thing took me 18:30 to complete. I also did a 10 minute cool down walk around the block. I'm spent!!! I'll try again Sunday morning but with squats and crunches. Nest time I'll try and take a video to see how pathetic I look running.