Where is VirtualBox's virtual hard disks repository?

For Microsoft Windows OS, default Storage is in %HOMEDRIVE%%HOMEPATH%\VirtualBox VMs

Which typically expands out to

C:\Users\[username]\VirtualBox VMs

The subfolders will be named with the Name: field contents entered in the Create Virtual Machine dialog where you're asked to enter Name and OS information.

If you've decided to do a little more orgainization, the Machine Name subfolders will be under the Machine Group subfolder. Full implementation may look like the following:

C:\Users\[username]\VirtualBox VMs\[vm group]\[vm name]

The VDI may exist elsewhere if you used the option to Use an existing virtual hard drive

It's easy to find where the VDI disks reside by using File=>Virtual Media Manager

At the risk of being scolded for not answering the EXACT question...

On Ubuntu (and other Linux distributions?), the directory is:

/home/[USER NAME]/.config/VirtualBox