Unable to create the virtual directory. The URL ... is already mapped to a different folder
I wasn't quite sure what I'd changed to cause this to start happening. I'd get this error when I was opening my solution. My web project wouldn't even load. I had to edit the csproj file manually and change my project to use "localhost".
This would allow my project file to open, but it wouldn't solve the problem. When I changed it back from LocalHost it would just break again.
SOLUTION:
I found that my IISExpress had been cluttered up by past configurations and projects. I had to clean up my applicationhost.config file, which is located here:
<UserProfile>\Documents\IISExpress\config
I just searched this file for my project folder path and removed sites that referenced that same path. Once I did that, I could change my project back to something other than LocalHost and everything worked fine.