Disney Dollars are so cute! You can pick them up in WDW or at your local Disney Store. They are basically just 'dollar' bills, but with Disney characters on them. For example, I have a $10 Stitch and a $5 Aurora laying right here.
Each year the characters change. Stitch is from 2005, and Aurora is from 2007. Just like real dollars, there are many variations that appeal to collectors. You can learn all about them by visiting
http://www.disneydollars.net/index.php.
You will find information on that site stating that they are also available by mail order, if you'd so choose. Otherwise you can easily pick them up at your resort when you check in.
Is it worth it? Well, if you buy them at Disney then they cost only face value (no charge for shipping), so yes, I'd say they are worth it. They're very cute, and a lot more fun than our everyday money (or a gift card). Not necessary, but they do add a little extra pixie dust. And who couldn't use that? :)