I've been interested in the bodyblade proprioceptive training device for some time. My dear friend and colleague, Sarah Walker brought one to Tai Chi camp in Wakulla Springs one year. They come up on ebay, and I found one for significantly less than the usual price, bwa ha ha ha!

Essentially a springy piece of fiberglass about 48" X 2" X 1/8" with weighting on the tips, and a handle in the middle. When you get it bouncing back and forth - in a myriad of different positions - is maximally stimulates the stabilizer muscles and joint proprioceptors throughout the body.

Used in conjunction with balance equipment - rock boards, balance balls, etc - it can be really useful at improving balance and over proprioception. Which should also translate to better reaction time. I also suspect it can help with training fast twitch muscles as well.