Where is the IIS Express configuration / metabase file found?

For VS 2015 & VS 2017: Right-click the IIS Express system tray icon (when running the application), and select "Show all applications":

Then, select the relevant application and click the applicationhost.config file path:

The configuration file is called applicationhost.config. It's stored here:

My Documents > IIS Express > config

usually, but not always, one of these paths will work

%userprofile%\documents\iisexpress\config\applicationhost.config
%userprofile%\my documents\iisexpress\config\applicationhost.config

Update for VS2019
If you're using Visual Studio 2019+ check this path:

$(solutionDir)\.vs\{projectName}\config\applicationhost.config

Update for VS2015 (credit: @Talon)
If you're using Visual Studio 2015-2017 check this path:

$(solutionDir)\.vs\config\applicationhost.config

In Visual Studio 2015+ you can also configure which applicationhost.config file is used by altering the <UseGlobalApplicationHostFile>true|false</UseGlobalApplicationHostFile> setting in the project file (eg: MyProject.csproj). (source: MSDN forum)

Since the introduction of Visual Studio 2015, this location has changed and is added into your solution root under the following location:

C:\<Path\To\Solution>\.vs\config\applicationhost.config

I hope this saves you some time!