Google's Datacenters on Punch Cards
what-if.xkcd.com15 exabytes is way more than I'd intuitively guess, but he does make a point in his guesstimations.
It makes you wonder what is the capacity of S3. Does Amazon reveal this number? In any case, Amazon will still have much more storage capacity between EBS, CloudFront caching, and DynamoDBs.
The thing I found particularly fascinating about this one is the mining of public data, and the occasional slip, to come up with a defensible, if not plausible, estimate of Google's capacity.