Amazon S3
Amazon's new S3 storage service (thanks Mike Arrington) is very exciting. Basically Amazon provides backed up storage at 15 cents per GB per month, 20 cents per GB transferred. Your bits are accessible to yourself through a REST or SOAP API and to your users via HTTP, URL-authenticated HTTP and BitTorrent; they're planning other transfer methods.
A lot of people actually underestimate how difficult it is to provision and back up large amounts of storage, even with tools like MogileFS; if we started coding 23 today we would seriously consider using S3 for primary storage.
One piece that's missing would be the ability to place servers or processing power close to an Amazon location, something that they understood with Alexa Web Services. The current design of S3 is perfect for serving large amounts of content, but less ideal if any processing is necessary; for example consider the example where users upload video files, but we need to do some post-processing on the video at a later date. This would involve downloading huge files from Amazon, doing the processing, and uploading them again, where it would probably be more ideal just to run the code on Amazon's servers. (Like Alexa!)
It will be exciting to see how S3 plays in the blogosphere. I could imagine a certain amount of skepticism, and I'll try to address that when I see the response.