Platform as a Service (PaaS) allows businesses launching an application to be blind to the infrastructure their app is running on, and focus solely on the application they are launching. PaaS providers often include everything from a development and testing environment to hosting in an elastic cloud environment.
For small businesses, PaaS can offer huge cost savings through reduced development time. Imagine that Infrastucture as a Service (IaaS) is akin to electricity, and Software as a Service(SaaS) is similar to something that consumes that commodity-- let's say, a blender. PaaS falls somewhere in between the two. For the business that is building the blender, development time, resources and the Platform itself can become their competitive advantage.
It's arguable that many IaaS providers offer aspects of PaaS, like the Rackspacecloud, Amazon EC2, or Terremark's VCloud offering, but the following is a rundown of a few PaaS solutions that provide "soup to nuts" capabilities -- from development to launch. Additionally, since vendor lock-in is one of the most prevalent concerns to businesses considering cloud computing solutions, we'll divide the platforms on that basis. For more on the issue of portability inside the cloud, take a look at CIO.com's article on the issue.
Portable (Open) PaaS
The criteria for portable - or open - PaaS is not necessarily related to open source, it's merely the answer to the question: Can we develop a web app, and then move it to another Cloud or competitors' hosting solution with relative ease?
Cloud Foundry builds upon the open source Cloud Tools, and offers a simple development environment and deployment platform for Spring, Java, and Grails applications (you can get more details on it here). While Cloud Foundry is targeted towards developers planning to launch their application into Amazon's EC2 Cloud, its basis in Cloud Tools provides the flexibility to deploy in to any VMware Vsphere infrastructure.
Heroku offers Ruby developers the ability to push their applications to the Heroku platform using GIT-- an open source version control system. Once you push your code to the server, it compiles a read-only slug, and allocates resources as needed across the cloud. I consider this an open platform, because of the emerging Ruby-specific PaaS market, including Morph Labs, Engine Yard, and PoolParty. Read DigitalHobbit's more in-depth review of some of these Ruby-specific PaaS offerings.
The converse of the commentary above on "Open" systems applies here. It's arguable that one could port their app off of one of the following platforms, but the following are platforms that have significant barriers to migrating your app to a competitor.
Azure is Microsoft PaaS offering. Building on the current developer base of .NET and Visual Studio, Azure aims to build on existing technologies and skills. Microsoft calls it their cloud operating system, but I find it a bit easier to understand if you imagine developing in .NET and launching the application in to a Microsoft managed data center. If you're still curious, Allbusiness has a good article on "Demystifying Azure".
Force.com, is the same platform that the very popular SaaS Salesforce runs on. Developers familiar with Java or C# will find it easy to get started. The Force Cloud seems well suited for businesses already reliant upon the SalesForce CRM, or who are looking to develop business applications that could easily integrate with it. However, word is that this fall VMForce, a partnership between VMWare and SalesForce, would allow Java Developers to deploy to the Force Cloud via the Spring IDE, giving it similar functionality to the open Cloud Foundry Platform.
Intuit's Platform Partner is Intuit's PaaS targeted at businesses building SaaS that can take advantage of both an easy conduit to Quickbooks data, access to Intuit's IaaS, and the opportunity for direct marketing of their SaaS to Ituit's large customer base. In addition, Intuit and Microsoft have partnered to also allow Azure apps to be marketed to the Intuit Workplace App Center. Read more on the partnership at Intuit's Blog.
This is by no means an exhaustive list, so please chime in if there are any platforms that your business has found useful, and let us know if you're "locked-in" to any particular IaaS, or PaaS.