Having multiple Excel instances launched, how can I get the Application Object for all of them?

To list the running instances of Excel:

#If VBA7 Then
  Private Declare PtrSafe Function AccessibleObjectFromWindow Lib "oleacc" ( _
    ByVal hwnd As LongPtr, ByVal dwId As Long, riid As Any, ppvObject As Object) As Long

  Private Declare PtrSafe Function FindWindowExA Lib "user32" ( _
    ByVal hwndParent As LongPtr, ByVal hwndChildAfter As LongPtr, _
    ByVal lpszClass As String, ByVal lpszWindow As String) As LongPtr
#Else
  Private Declare Function AccessibleObjectFromWindow Lib "oleacc" ( _
    ByVal hwnd As Long, ByVal dwId As Long, riid As Any, ppvObject As Object) As Long

  Private Declare Function FindWindowExA Lib "user32" ( _
    ByVal hwndParent As Long, ByVal hwndChildAfter As Long, _
    ByVal lpszClass As String, ByVal lpszWindow As String) As Long
#End If

Sub Test()
  Dim xl As Application
  For Each xl In GetExcelInstances()
    Debug.Print "Handle: " & xl.ActiveWorkbook.FullName
  Next
End Sub

Public Function GetExcelInstances() As Collection
  Dim guid&(0 To 3), acc As Object, hwnd, hwnd2, hwnd3
  guid(0) = &H20400
  guid(1) = &H0
  guid(2) = &HC0
  guid(3) = &H46000000

  Set GetExcelInstances = New Collection
  Do
    hwnd = FindWindowExA(0, hwnd, "XLMAIN", vbNullString)
    If hwnd = 0 Then Exit Do
    hwnd2 = FindWindowExA(hwnd, 0, "XLDESK", vbNullString)
    hwnd3 = FindWindowExA(hwnd2, 0, "EXCEL7", vbNullString)
    If AccessibleObjectFromWindow(hwnd3, &HFFFFFFF0, guid(0), acc) = 0 Then
      GetExcelInstances.Add acc.Application
    End If
  Loop
End Function

This would be best as a comment on Florent B.'s very useful function that returns a collection of the open Excel instances, but I don't have sufficient reputation to add comments. In my tests, the collection contained "repeats" of the same Excel instances i.e. GetExcelInstances().Count was larger than it should have been. A fix for that is the use of the AlreadyThere variable in the version below.

Private Function GetExcelInstances() As Collection
    Dim guid&(0 To 3), acc As Object, hwnd, hwnd2, hwnd3
    guid(0) = &H20400
    guid(1) = &H0
    guid(2) = &HC0
    guid(3) = &H46000000
    Dim AlreadyThere As Boolean
    Dim xl As Application
    Set GetExcelInstances = New Collection
    Do
        hwnd = FindWindowExA(0, hwnd, "XLMAIN", vbNullString)
        If hwnd = 0 Then Exit Do
        hwnd2 = FindWindowExA(hwnd, 0, "XLDESK", vbNullString)
        hwnd3 = FindWindowExA(hwnd2, 0, "EXCEL7", vbNullString)
        If AccessibleObjectFromWindow(hwnd3, &HFFFFFFF0, guid(0), acc) = 0 Then
            AlreadyThere = False
            For Each xl In GetExcelInstances
                If xl Is acc.Application Then
                    AlreadyThere = True
                    Exit For
                End If
            Next
            If Not AlreadyThere Then
                GetExcelInstances.Add acc.Application
            End If
        End If
    Loop
End Function

@PGS62/@Philip Swannell has the correct answer for returning a Collection; I can iterate all instances; and it is brilliant, as @M1chael comment.

Let's not confuse Application objects with Workbook objects... ...Of course it would be possible to write a nested loop that loops over the workbooks collection of each application object

This is the nested loop implemented and fully functional:

Sub Test2XL()
  Dim xl As Excel.Application
  Dim i As Integer
  For Each xl In GetExcelInstances()
    Debug.Print "Handle: " & xl.Application.hwnd
    Debug.Print "# workbooks: " & xl.Application.Workbooks.Count
    For i = 1 To xl.Application.Workbooks.Count
        Debug.Print "Workbook: " & xl.Application.Workbooks(i).Name
        Debug.Print "Workbook path: " & xl.Application.Workbooks(i).path
    Next i
  Next
  Set xl = Nothing
End Sub

And, for Word instances, the nested loop:

Sub Test2Wd()
  Dim wd As Word.Application
  Dim i As Integer
  For Each wd In GetWordInstancesCol()
    Debug.Print "Version: " & wd.System.Version
    Debug.Print "# Documents: " & wd.Application.Documents.Count
    For i = 1 To wd.Application.Documents.Count
        Debug.Print "Document: " & wd.Application.Documents(i).Name
        Debug.Print "Document path: " & wd.Application.Documents(i).path
    Next i
  Next
  Set wd = Nothing
End Sub

For Word you have to use what is explained in the end of this thread