As far as I'm concerned, the Peacock feather is simply 'Magic!' It's irridescence makes it irresistable to many fish and is used in a variety of patterns.
I couldn't imagine a fly tying desk that didn't have an ample supply of peacock 'herl.' These are the long thin fibers which stick out from the stem of the tail feather and also surround the eye. You can see it around the eye in the photo on the left. On the right is 'strung' herl, which is the most common way to buy it. It has already been stripped off the stem and then bound for easy access and less mess.