This may be old hat to some folks but for the others I hope you find it useful; particularly when doing unit tests.
I have a project we will call "MainApp" and another project "MainAppTest" that is responsible for unit testing MainApp. Some of the classes I am testing depend on some application configuration settings existing in the app.config file of the MainApp - but I didn't want to have to keep a copy of app.config in MainAppTest (if you don't have an app.config in the test project the app settings won't be found when you try to load them).
What to do? I don't want to copy app.config what I really want is for my test project to "point" at the App.config in the MainApp project. It turns out this is possible and actually pretty easy to do even if the option is a little hidden.
Using VS 2005 just right click on the project you want to add the file to, select "Add -> Existing Item" and then browse for the file you want to "point" to once the open file dialog appears. However, before you click the "ADD" Button notice that on the right side of the button is a little arrow. Click on it and you will get two options: "Add" and "Add as Link" - select the second "Add as Link" option and you'll be good to go.
Now the file will be included in your project but the file icon will have a small arrow overlay.
If you want to remove the file link from your project just right click on the linked file, selected remove, and bam! You're done. The file will still exist in the original project but it won't be in your alternate project anymore.