Find the SQL Server Product Key with the Right Script

For SQL Server 2005

USE master
GO
EXEC xp_regread 'HKEY_LOCAL_MACHINE','SOFTWAREMicrosoftMicrosoft
SQL Server80Registration','CD_KEY'
GO

For Sql Server 2008 R2

USE MASTER
GO
EXEC XP_REGREAD 'HKEY_LOCAL_MACHINE','SOFTWAREMicrosoftMicrosoft SQL Server100BIDSSetup','ProductCode'
GO

For SQL Server 2012:

USE MASTER

GO

EXEC XP_REGREAD