2. BOSU Ball: attack that core with this half-sphere that you can use "both sides up", and slip this under the bed or under the couch when not in use. Bicycle kick crunches on this is certified torture.
3. Fold-A-Way Rowing Machine: An oldie but a goodie. You're not going to get ripped using this, but it's easy to store away when not in use and offers resistance and incline options for a healthy cardio workout.
4. Universal Door Mount Chin up Bar: This is about the best single piece of workout equipment you can get if space and budget is an issue. Chin ups, pull ups, leg raises, one-arm chinups, side crunches...all you need is a doorway and your own body weight to keep this bar in place. Great for stretching out your back too.
5. Nintendo Wii Sports: Don't laugh...spend an hour doing Wii Boxing, and you'll likely be huffing-puffing and sitting inbetween rounds while burning 150 calories per 15 minutes swinging wildly, all the while having a blast. Play tennis, baseball, golf, bowling and boxing in the comfort of one's living room...but close the blinds....you'll look silly (I know I do). And finding a Wii might be an exercise in frustration in itself.