The Camwood bat is the new swing trainer on the market. I have one and use it for my two sons - ages 11 and 12 and I can honestly report it has slightly increased their swing speed as gauged by a Blast Motion sensor.
You can get the adult version below.
Here is a link to the youth size Camwood bat.
Here's a good breakdown of the bat from Northern Baseball Training
And here's the promotional video from the Camwood Bat company.
Have you used one?
What do you think about it?