Comparing two versions of a spreadsheet (old versus new), I wanted to have a column of the current row number on the old spreadsheet and the corresponding row number on the new one.
For the old spreadsheet, this would just be the row number e.g. column 3 has number 3.
An easy way to do this is to type two numbers in the first two rows (i.e. 1 and 2) and then highlight them as if to drag them which causes the little box to appear in the bottom right hand corner of the cell. Then drag the box down the rest of the column.
When you do this, Excel works out the relationship between the two numbers (i.e. row x = x; row x+1 = x+1) and then applies this to the rest of the column.