You really have to know what you want to do - what kind of website, scale, usage model, etc. - and investigate the leading Cloud platforms on your own. Most provide trials (you have to prove you're 'serious', for some). They all post technical information and most have pricing on their sites. Here's a few with thoughts off the top of my head. They are NOT all created equal as far as deploying your app. All of them are targeting website hosting and dynamic excess capacity for those websites ("elasticity") for sure.
Amazon EC2: Probably best choice for basic, developer oriented, less secure websites and applications with very flexible, hourly pricing
Google: Great dev tools and largest platform -- but proprietary and less flexible, smaller apps
SaleForce: More proprietary, but new tools and known performance/functionality, more of an enterprise focus
Rackspace: Many locations and great service- good pricing close to Amazon, enterprise focus, but promising
3Tera: Many locations - based on established datacenters, early in the Cloud
GoGrid: Very promising - own their datacenters
Joyent: Good service, monthly charges
Terremark: Enterprise cloud - very secure and well managed with several massive datacenters
IBM: Talk to a rep about BlueCloud - clearly will be a dominant force before long (HongKong just opened yesterday)
AT&T: Enterprise customer focus... lots of datacenters
Verizon: Not much noise lately, but expect them to be a major player.