The Powerball is an incredibly useful training aid. I can't fully explain how good this thing is - it's something you need to have a go at, and see for yourself.
Operating the Powerball may take a bit of practice, and the technique takes a while to get used to - but it's worth it.

Basically, it's a ball you hold in your hand, which has a gyroscope inside it. You 'start it up', but inserting a small cord into the centre and then tugging it away quickly - a bit like starting a speed boat motor. With experience, you can start the device by flicking it with your finger - but this takes time to learn.
As the inter ball rotates, you roll your wrist round in circles to keep it going - this will build up momentum, gradually making the Powerball harder to hold.

The good thing about this product, is that the better you are, the harder it is - therefore, you can never really beat it.
Powerballs are great for any sport that involves the wrist, e.g. golf, badminton, squash & tennis. As such, many top athletes use them as a training device.
Costing around £14 - £80, there are a few different types available. Most have a small lcd display on the top, which can measure the highest rpm achieved, and other stats. This helps maintain the fun, because you'll find yourself frequently coming back to try and beat your top score.

I've got a couple of Powerballs - with my favourite being the all-metal version - which provides a very difficult workout.

Although Powerballs are well built, they need to be treated with care, and should never been dropped or shaken around when in motion - this can damage the device. Also, Beware of fake replicas - For more info, look at the Powerball website at: www.powerball-gyros.com/

Not everyone will like them - but they're a great training aid, and fantastic for people who are recovering from wrist injuries. Highly Recommended