Amazon Web Services is offering two relatively new services: ubiquitous storage via S3 and ‘elastic’ computing via EC2. The Cardbox folks have an in-depth analysis of S3 online, and Smugmug uses an S3 backend.This is very interesting. If it wasn’t for all the problems with the S3 contract, I would consider using this.
Now; that being said, ParkPlace is worth looking at – it allows running an S3-compatible server on any of your machines. Here’s some more info on ParkPlace.
And finally, here’s a good overview of S3 compatible software.