The best cardio exercise I know is the swing kettlebell exercise. But if you don't have a KB, then you could probably replicate it by putting a few heavy weights (improvise, use a few cans if you have no 'weight' weights) in a pillowcase, tie it tight, and swing.

That guy in the video isn't doing any of what she's saying. He is concentrating only on not whacking his nuts. I am going to give that a try though.

Jumping rope can give you an intense cardio workout. Since it is very rare to be able to jump rope for even 10 minutes straight (try it!), it works well as an interval workout, with marching in place in between 1-2 minute bursts of fast jumping.

Put on some music and dance while cleaning. Or while avoiding cleaning. There's also some cardio workout videos on YouTube, like kickboxing or yoga, short 5-minute videos.

If you have any stairs in your home, you could use them for a cardio-step workout. Grab a bottle of water for each hand and do some strength training while you're stepping up and down and up and down.

SparkPeople has a bunch of strength-training exercises you can do at home without equipment, but not really any cardio stuff. Bonus: there's a MetaFilter team. It was mentioned on MeTa a few months ago.

I love to put on some great music and dance around the house like a madwoman, singing at the top of my lungs. That always gets me sweating.

I have a treadmill you can have if you're in Chicago.

I need some good exercises for apartment dwellers. I don't know that I can get away with jumping rope with neighbors downstairs. Guess it's back to sit-ups.

When I couldn't afford a gym, I ran up and down the stairs in our apartment building. Sometimes I would stop on a landing and do squats and lunges and suchlike.