If you've got a smaller space or you're just looking for one piece of equipment to strengthen your entire body, you've got to try the TRX All-in-One Suspension Training System ($150). You can focus on a specific body part or do a full-body workout with the TRX, and the best part is, this suspension system is so small, you can pack it in your bag and train anywhere.
Here's a beginner full-body TRX workout to get you started.