# Example with Get-Process
Get-Process |
  Select-Object -Property '#', ProcessName |
    ForEach-Object -begin { $i = 0 } -process {
      $_.'#' = $i++
      $_
    }
# You can start at 1 if you prefer
Get-service |
  Select-Object -Property '#', Name |
    ForEach-Object -begin { $i = 1 } -process {
      $_.'#' = $i++
      $_
    }