Fixed version of https://gallery.technet.microsoft.com/scriptcenter/Get-product-keys-of-local-83b4ce97/view/Discussions#content

Get-ProductKeyv2.ps1

function Get-ProductKey {
     <#   
    .SYNOPSIS   
        Retrieves the product key and OS information from a local or remote system/s.
         
    .DESCRIPTION   
        Retrieves the product key and OS information from a local or remote system/s. Queries of 64bit OS from a 32bit OS will result in 
        inaccurate data being returned for the Product Key. You must query a 64bit OS from a system running a 64bit OS.
        
    .PARAMETER Computername
        Name of the local or remote system/s.
         
    .NOTES   
        Original Author: Boe Prox
	Components taken direct from Jakob Bindslet ([email protected])
	Small changes to make keys correct - Jordan Rinke
        Version: 1.1       
            -Update of function from http://powershell.com/cs/blogs/tips/archive/2012/04/30/getting-windows-product-key.aspx
            -Added capability to query more than one system
            -Supports remote system query
            -Supports querying 64bit OSes
            -Shows OS description and Version in output object
            -Error Handling
     
    .EXAMPLE 
     Get-ProductKey -Computername Server1
     
    OSDescription                                           Computername OSVersion ProductKey                   
    -------------                                           ------------ --------- ----------                   
    Microsoft(R) Windows(R) Server 2003, Enterprise Edition Server1       5.2.3790  bcdfg-hjklm-pqrtt-vwxyy-12345     
         
        Description 
        ----------- 
        Retrieves the product key information from 'Server1'
    #>         
    [cmdletbinding()]
    Param (
        [parameter(ValueFromPipeLine=$True,ValueFromPipeLineByPropertyName=$True)]
        [Alias("CN","__Server","IPAddress","Server")]
        [string[]]$Computername = $Env:Computername
    )
    Begin {   
        $map="BCDFGHJKMPQRTVWXY2346789" 
		$hklm = 2147483650
		$regPath = "Software\Microsoft\Windows NT\CurrentVersion"
		$regValue = "DigitalProductId"
    }
    Process {
        ForEach ($Computer in $Computername) {
            Write-Verbose ("{0}: Checking network availability" -f $Computer)
            If (Test-Connection -ComputerName $Computer -Count 1 -Quiet) {
                Try {
                    Write-Verbose ("{0}: Retrieving WMI OS information" -f $Computer)
                    $OS = Get-WmiObject -ComputerName $Computer Win32_OperatingSystem -ErrorAction Stop                
                } Catch {
                    $OS = New-Object PSObject -Property @{
                        Caption = $_.Exception.Message
                        Version = $_.Exception.Message
                    }
                }
                Try {
                    Write-Verbose ("{0}: Attempting remote registry access" -f $Computer)
					$remoteReg = [WMIClass]"\\$Computer\root\default:stdRegProv"
					$data = $remoteReg.GetBinaryValue($hklm,$regPath,$regValue)
					$value = ($data.uValue)[51..66]
                    $ProductKey = ""  
                    Write-Verbose ("{0}: Translating data into product key" -f $Computer)

                    for ($i = 24; $i -ge 0; $i--) { 
                      $r = 0 
                      for ($j = 14; $j -ge 1; $j--) { 
                        $r = ($r * 256) + $value[$j]
                        $value[$j] = [math]::Floor([double]($r/24)) 
                        $r = $r % 24
                      } 
                      $ProductKey = $map[$r] + $ProductKey
                      if (($i % 5) -eq 0 -and $i -ne 0) { 
                        $ProductKey = "-" + $ProductKey 
                      } 
                    }
                } Catch {
                    $ProductKey = $_.Exception.Message
                }
                $object = New-Object PSObject -Property @{
                    Computername = $Computer
                    ProductKey = $ProductKey
                    OSDescription = $os.Caption
                    OSVersion = $os.Version
                } 
                $object.pstypenames.insert(0,'ProductKey.Info')
                $object
            } Else {
                $object = New-Object PSObject -Property @{
                    Computername = $Computer
                    ProductKey = 'Unreachable'
                    OSDescription = 'Unreachable'
                    OSVersion = 'Unreachable'
                }  
                $object.pstypenames.insert(0,'ProductKey.Info')
                $object                           
            }
        }
    }
}