Pretty Clouds
When I was at I Love Rewards we need to keep costs low, but we also needed scalability. When we re-wrote our enterprise platform, our Director of I.T. at the time (Amin Lalji) got us in on the beta of Amazon's EC2 (Elastic Compute Cloud). We were one of the first people to run a cluster on it, which was pretty cool. You pay per CPU-hour. Unfortunately, we later determined that running a cluster for 10 concurrent users didn't make much sense. Fortunately, we had spent some time architecting things to be abstract & swappable, so scaling back to a single box was easy. When I left, traffic was getting high enough that we were about to upgrade to two servers (1 for the database, and one for the application). I'm sure that has been easy for them, and that it'll be easy to scale back up to a cluster when the time comes.
We also stored all of our images on Amazon's S3 (Simple Storage Service). As much as possible lived in the Cloud.
At my new job, we ain't got no clouds... but we're about to. This time, however, it won't be Amazon. It looks like we'll using Nirvanix. Unlike Amazon, they have a nice Zend_Framework_Nirvanix class for accessing their web services. Why go with them and not S3? The big reason is that on top of simply offering storage, they offer some VERY handy multimedia services. Specifically, they do image resizing, video transcoding from one format to another, and video frame extraction. Pretty hot stuff. Their pricing is good, too. You pay a certain rate to store data on their network, and either $1/GB pay-as-you-go, or $0.20/GB to have all your content transcoded & frame-extracted! Nice 
I'm looking forward to playing with it!
What can Nirvanix' services do for you? (no, I'm not getting paid for this)