Debra Dalgleish shares a manual technique and a code solution:
http://contextures.com/xlDataEntry02.html
> I need a macro to copy the first cell in a row down the column until it runs
> into a existing value and then copy that value down until it meets another
> existing value and then copy that value down the column and so on.

Signature
Dave Peterson
Hi,
Please make a back up copy of your workbook before testing. This macro
assumes that the data you want to copy is in column A.
Option Explicit
Dim RowCount As Double
Dim Iloop As Double
Sub CopyDown()
'Turn off warnings, etc.
Application.ScreenUpdating = False
Application.DisplayAlerts = False
RowCount = Cells(Rows.Count, "A").End(xlUp).Row
For Iloop = 1 To RowCount
If IsEmpty(Cells(Iloop, "A")) Then
Cells(Iloop, "A") = Cells(Iloop - 1, "A")
End If
Next Iloop
''Turn on warnings, etc.
Application.DisplayAlerts = True
Application.ScreenUpdating = True
End Sub

Signature
Ken Hudson
> I need a macro to copy the first cell in a row down the column until it runs
> into a existing value and then copy that value down until it meets another
> existing value and then copy that value down the column and so on.