My BOSU balance ball.
When I first started committing myself to my sport and training, I was introduced to a BOSU. Since then, it's been my favorite fitness tool to use and train with. I've heard that Costco has them on sale for $70, but expect to spend around $100 for one of these suckers. Once you have one, get comfortable with it by simply standing on the uneven surface, moving into squats as you feel ready. After a few sessions, try using the trainer for lunges, push-ups, and step-ups for multi-tasking exercises that improve balance, help shape your core, and activate fast-twitch muscles for better response times.
Stuck in a BOSU rut? Check out this list of 41 awesome BOSU exercises.