I have been using Nirsoft's ProduKey. It works with Windows 8.

Answer from Erik on Stack ExchangeProduKey is a small utility that displays the ProductID and the CD-Key of Microsoft Office (Microsoft Office 2003, Microsoft Office 2007), Windows (Including Windows 7 and Windows Vista), Exchange Server, and SQL Server installed on your computer.
Windows 8 Product Keys
Where can I find Windows 8 product key after reinstall?
How to obtain Windows product key online
Windows 8.1 Product Key
I'm always working on PC's that need Windows 8, we don't have an OEM copy in store, just burned versions. I don't know who did this or why but they didn't write down any product key, is there someone I can get a key from?
I have been using Nirsoft's ProduKey. It works with Windows 8.

ProduKey is a small utility that displays the ProductID and the CD-Key of Microsoft Office (Microsoft Office 2003, Microsoft Office 2007), Windows (Including Windows 7 and Windows Vista), Exchange Server, and SQL Server installed on your computer.
A utility has been released over at mydigitallife.info for this purpose
Windows 8 Product Key Viewer works for both version of Windows as 32-bit and 64-bit and it can
Decodes Windows Product Keys.
Decodes product keys with N character.