WebMatrix is a 15MB download (50MB if you don’t have .NET 4 installed) and is quick to install.

The 15MB download includes a lightweight development tool, IIS Express, SQL Compact Edition, and a set of ASP.NET extensions that enable you to build standalone ASP.NET Pages using the new Razor syntax, as well as a set of easy to use database and HTML helpers for performing common web-tasks.  WebMatrix can be installed side-by-side with Visual Studio 2010 and Visual Web Developer 2010 Express.

Note: Razor support within ASP.NET MVC applications is not included in this first beta of WebMatrix – it will instead show up later this month in a separate ASP.NET MVC Preview - which will also include Visual Studio tooling support for it.