Looking through the achievement list for Visual Studio 2010, it says that there is an achievement for having 3 startup projects.
How do you have multiple startup projects at once? Whenever I right click a project and select "Set as StartUp Project" it switches my current startup project to the new one. I wasn't aware multiple startup projects was even a possibility.
How to: Set Multiple Startup Projects:
Visual Studio allows you to specify how more than one project is run when you start the debugger.
To set multiple startup projects
In Solution Explorer, select the solution.
On the Project menu, click Properties. The Solution Property Pages Dialog Box opens.
Expand the Common Properties node, and click Startup Project.
Click Multiple Startup Projects and set the project actions. For more information, see Startup Project, Common Properties, Solution Property Pages Dialog Box.