What's the best way to get an effective cardio workout at home?

So far I'm doing weights and walking/running 2 miles a day at the gym. I'd like to be doing 4 or 5 miles but I never have enough time to stay and complete it.

Are there any good workouts I can do at home to makeup for the addition 2-3 miles?
What's the best way to get an effective cardio workout at home?
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