Sum of row n through last row

If you want something that just works in Google Spreadsheets (as the title suggests), you can use open-ended ranges:

=SUM(A2:A)

In Excel, you can specify the maximum rows for that version; for example, for 2007 and 2010:

=SUM(A2:A1048576)

This will work in Google Spreadsheets as well, and is beyond the current theoretical row limit in GSheets (400000)*.

Edit: *The quoted limit of the new version of Sheets is actually now 2 million cells.

Try this

=SUM(OFFSET(A1,1,0,ROWS(A:A)-1,1))

Works in both Excel and Google Docs

One downside: by using OFFSET the formula becomes Volatile ie is calculated every time the sheet calculates irrespective of data change.

One way you can do this in both Excel/Google Spreadsheets is to add a dummy column with a single value at the top (and all blanks for the remainder of the column).

You can then do a SUMIF to summarize all values except the one which is not "" - which should just be the cell A1 - put the following in B1, with whatever numbers below B1 you want and anything in A1.

=SUMIF(A:A,"",B:B)