Here is an example of a big Visual Studio.NET solution that contains numerous projects:
Such a big Visual Studio.NET solutions can be organized by grouping related projects into folders. Just right-click on the solution node and choose Add | New Solution Folder to add a Solution Folder. Then just Drag'n'Drop projects into this folder.
This also will allow you to do some operations on a group of projects. For example, unload all projects in a Solution Folder, collapse or hide entire Solution Folder so that you can work easily in Solution Explorer in Visual Studio.NET.