I don't know how long it's been around, but I found a nice organization feature for VS Solutions, Solution Folders. Solution Folders are logical (not physical) folders that you can use to organize projects and solution items. You can nest folders as deep as you like also, just right-click the solution in the Solution Explorer and click "Add->New Solution Folder".
It's easy to get carried away, but I like to organize my projects by app, when I have over 10 or so projects in my solution. Typically, it will look like this:
- Common Projects
- Windows App
- Web App
Here's an example with NBehave:
This solution was small enough that it really didn't need it, but with a solution with several dozen projects and many applications, solution folders can prevent quite a few headaches.