XAML Regions extension by Jacob Johnston helps you organize xaml code by grouping relevant elements and speeds up navigation by collapsing less important blocks of markup. You can create a region with two comment lines containing region/endregion keywords and then collapse and expand it as needed:

Region definition in comments for XAML editor in Visual Studio 2010

Collapsed region in XAML editor

You can use any convention for the region keyword in a comment (region, Region, #Region), same with endregion (endregion, End Region, #EndRegion). The text after the region keyword forms visible region description.

The only suggestion I have for XAML Regions is to add a context menu command to the XAML editor to surround selection with a region (or add two commands to create and delete the region).

You can download the free XAML Regions extension for Visual Studio 2010 from Visual Studio Gallery.