Microsoft Web platform installer is a tool used to install Microsoft’s entire web platform, including IIS, Visual Web Developer 2008 Express Edition, SQL Server 2008 Express Edition, the .NET framework, ASP.NET MVC, Visual Studio tools for Silverlight. We can choose to install specific products in the the User interface. It also keeps the products up to date by offering us to install the latest updates on each of the installed Products. It can be downloaded here.
Microsoft Web Application installer is used to install the most freely used and popular ASP.NET and PHP Web applications available like Graffiti, DotNetNuke, WordPress, Drupal, OSCommerce etc. It is still in Beta Stage. It can be downloaded here.
References: Microsoft Web Platform Installer Page
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