amazon's elastic compute cloud
Now this is interesting, especially when combined with Amazon's Simple Storage Service:
Amazon Elastic Compute Cloud (Amazon EC2) is a web service that provides resizable compute capacity in the cloud. It is designed to make web-scale computing easier for developers. ... Amazon EC2 presents a true virtual computing environment, allowing you to use web service interfaces to requisition machines for use, load them with your custom application environment, manage your network's access permissions, and run your image using as many or few systems as you desire.
You can upload your own image, or use one of their "preconfigured" instances, each of which is the equivalent of 1.7ghz Xeon with 1.75gb of RAM, 160gb of disk and 250mb/s of bandwidth. And of course it's integrated with S3. Pricing is $0.10 per instance-hour (around $72 per month).