In August, the Google Reader Blog posted a list of the most commonly used extensions in feeds. Here is my list of the top 10 most useful extensions for syndication: 10. iTunes This one really shouldn't even be on this list, but I include it because it did make a big splash when it was released. It also generated a lot of frustration and anger amoung podcast producers. The iTunes namespace extension was created to allow podcast feed producers to integrate their feeds with Apple's iTunes music player....