I bet you can guess that the a.co is for Amazon, k.co refers to the Kindle, and cloud.co will be used with Amazon's new cloud storage service. But the Z.co URL might be a stumper if not for Amazon's logo. Look at the logo again. Do you see how the smile/arrow starts at the 'A' and ends on the 'z'? That was a deliberate design decision because Amazon wanted the logo to imply "from A to Z". i bet that's why they bought the z.co URL.