Good question! As you point out, the different offerings fit into different categories:
EC2 is Infrastructure as a Service; you get VM instances, and do with them as you wish. Rackspace Cloud Servers are more or less the same.
Azure, App Engine, and Salesforce are all Platform as a Service; they offer different levels of integration, though: Azure pretty much lets you run arbitrary background services, while App Engine is oriented around short lived request handler tasks (though it also supports a task queue and scheduled tasks). I'm not terribly familiar with Salesforce's offering, but my understanding is that it's similar to App Engine in some respects, though more specialized for its particular niche.
Cloud offerings that fall under Software as a Service are everything from infrastructure pieces like Amazon's Simple Storage Service and SimpleDB through to complete applications like Fog Creek's hosted FogBugz and, of course, StackExchange.
A good general rule is that the higher level the offering, the less work you'll have to do, but the more specific it is. If you want a bug tracker, using FogBugz is obviously going to be the least work; building one on top of App Engine or Azure is more work, but provides for more versatility, while building one on top of raw VMs like EC2 is even more work (quite a lot more, in fact), but provides for even more versatility. My general advice is to pick the highest level platform that still meets your requirements, and build from there.
Not sure if this is still an issue but this resolved the issue for me. Just add the following to the Page_Load event of your Start Page:
protected void Page_Load(object sender, EventArgs e)
{
if (Request.UrlReferrer == null || string.IsNullOrEmpty(Request.UrlReferrer.AbsolutePath))
{
Session.Abandon();
FormsAuthentication.SignOut();
FormsAuthentication.RedirectToLoginPage();
}
}
Best Answer
IaaS, PaaS and SaaS are cloud computing service models.
IaaS (Infrastructure as a Service), as the name suggests, provides you the computing infrastructure, physical or (quite often) virtual machines and other resources like virtual-machine disk image library, block and file-based storage, firewalls, load balancers, IP addresses, virtual local area networks etc.
Examples: Amazon EC2, Windows Azure, Rackspace, Google Compute Engine.
PaaS (Platform as a Service), as the name suggests, provides you computing platforms which typically includes operating system, programming language execution environment, database, web server etc.
Examples: AWS Elastic Beanstalk, Windows Azure, Heroku, Force.com, Google App Engine, Apache Stratos.
While in SaaS (Software as a Service) model you are provided with access to application software often referred to as "on-demand software". You don't have to worry about the installation, setup and running of the application. Service provider will do that for you. You just have to pay and use it through some client.
Examples: Google Apps, Microsoft Office 365.
Few additional points regarding your question:
AWS (Amazon web services) is a complete suite which involves a whole bunch of useful web services. Most popular are EC2 and S3 and they belong to IaaS service model.
Although Hadoop is based on previous works by Google(GFS and MapReduce), it is not from Google. It is an Apache project. You can find more here. It is just a distributed computing platform and does not fall into any of these service models, IMHO.
Microsoft's Windows Azure is again an example of IaaS.
As far as popularity of these services is concerned, they all are popular. It's just that which one fits into your requirements better. For example, if you want to have a Hadoop cluster on which you would run MapReduce jobs, you will find EC2 a perfect fit, which is IaaS. On the other hand if you have some application, written in some language, and you want to deploy it over the cloud, you would choose something like Heroku, which is an example of PaaS.