Those of you who have done "Infidel", "Risa" or "Ramona" at CrossFit Koncepts know that an intense dose of cardio (i.e. chasing Dos or Cristine up and down the street) brings a WOD to a completely new level of suck. Or conversely, it can show you how much your conditioning has really progressed when you realize that you can perform at a very high intensity while under load (think 40 KB Push Presses with a 53 lbs KB as as "the rest" during a WOD). IF running isn't your "thing" then look into getting good with a jump rope; it's less jarring to the joints (when done correctly) and there is enough variety in the skills to keep you occupied for quite a while.
Dan John, who you've read about here on several occasions, weighs in with some of his favorite "cardio workouts". NOTE: they are all fast, brutal and effective... just how I like them!