If you want to experiment with the new extensibility options (MEF intregation, Wix deployment, WPF support,…) in VS 2010, download the Visual Studio 2010 SDK. Thanks to the removal of all the documentation from the binaries, it’s a much smaller download.
The Visual Studio 2010 SDK includes project templates that help developers create tool windows, menu commands, isolated shell projects, and editor extensions. The editor extensions include text adornments, colorizers, and margins. It also includes build tasks that help developers build and debug extensions. Building and debugging are managed in a second instance of Visual Studio named the experimental instance. The experimental instance provides a test bed for extensions without changing the primary installation of Visual Studio.