Undo VS 'Exclude from project'?

In the absence of an actual undo option you could select Add > Existing Item... to re-add the item you've just excluded.

Visual Studio Express 2012

Select Your Project Folder Click On "Show All Files" icon to show all Excluded Folders And Files

Then Right Click on Excluded Folder or File and Click on "Include In Project"

Make sure you are showing all files. There is a button at the top of the Solution Explorer called "Show All Files". To see this button, make sure that your project is selected in the solution explorer.

Show All Files

When this option is active, the file should be there, just grayed out. Right click it, and select "Include In Project".

Include In Project