Table Sum Powertoy for OneNote
As promised, here is the first of a week's worth of addins/powertoys for OneNote 2007. This addin will sum the columns of a table for you and add the new row to the bottom. It ignores non-numeric values in the column. It's only requirement is that you need to click in a table to select it before hitting the Sum button (although it will prompt you for clicking the table if you need to).
It does have some limitations. Inking (writing numbers in tables w/o converting them to text) is not supported, and tables within tables will cause problems.