Microsoft WebMatrix is a new web development tool that makes it easy for anyone to create a new web site. Start from any of the built-in or online templates or the Web Applications Gallery. Customize your site using the code and database editor. WebMatrix provides access to offers from Microsoft hosting partners, making it easy to choose the right host with a selection of hosting packages. Once a hosting package is configured, WebMatrix seamlessly publishes to the web.

It takes all the great features of the Web Platform Installer and adds development tools on top. If you just want to install one of many opensource packages on your windows server then the Web Platform Installer is for you. If you want to then edit those pages on your development machine and have that upload to a web hosting site then WebMatrix is the right tool.

Microsoft Web Matrix
http://www.microsoft.com/web/webmatrix/

Microsoft Web Platform Installer 3.0
http://www.microsoft.com/web/downloads/platform.aspx