$svc = [Microsoft.SharePoint.Administration.SPWebService]::ContentService
$col = New-Object -TypeName Microsoft.SharePoint.SPAlertTemplateCollection -ArgumentList $svc
foreach($a in $col)
{
    if ($a.Properties["Custom"] -eq $null)
    {
        continue
    }

    "=======================================================================" | Out-File log.txt -Append
    $a.Properties["FriendlyName"] | Out-File log.txt -Append    
    if (![System.String]::IsNullOrEmpty($a.Properties["NotificationMessage"]))
    {
        $a.Properties["NotificationMessage"] | Out-File log.txt -Append        
    }
}