I walk outside whenever I can, but when the weather's bad, I do either workout DVDs or free workouts from Exercise TV OnDemand. The Leslie Sansone ones are easy, but good if you do more than the one-mile one. I have a three-mile one that uses weights and lasts about 45 minutes, so that's a good workout. If I don't have as much time or energy, there are usually some two-mile ones on OnDemand that take 20-30 minutes.
I also have an old Tae Bo cardio DVD that is a great workout, and also Slim in 6, which is not much fun, but good for working out all the muscle groups.
My favorite workout DVD is actually a Dancing With the Stars one I got at Walmart for 10 bucks. It has three 15-minute dance workouts - swing, jive, and quickstep - plus a 5-minute cooldown. There are similar DVDs with other dances, but I really like this one. You learn a whole dance routine and get a great workout in 15 minutes. I am always sweating and out of breath by the end of one. You can combine them for a longer workout, but I usually need a break after one LOL. They're fun, though, if you like to dance!