This isn't a hack, but the way org-columns combines a collapsible tree view with spreadsheet like functionality is neat. I use it for project estimations where

This isn't a hack, but the way org-columns combines a collapsible tree view with spreadsheet like functionality is neat. I use it for project estimations where breaking things down further and further results in a tree, and the estimate sums of lower items in the tree are combined for the higher items. I don't know if this can be done in a spreadsheet.