I had the same bar on my last Prius too (and my Matrix before that), thats probably why I didn't bother mentioning it. Its made by Progress for the 03-08 Corolla or Matrix. It just tightens up the rear end and gets rid of a good amount of tail wag which I really don't like.
Installation is easier than easy, jack the rear of the car up, remove two bolts, pull the old bar out, put the new bar in and put the bolts back in. An impact helps because they're not tiny bolts, but its not absolutely necessary.
Here is an image for the sway bar. Mine is grey though.
