I think spreatsheets (nowadays Excel and Google Docs) are a much underappreciated tool. I neither fully appreciate nor fully understand them myself, but I'm starting to see the potential. Accountants and other business people already know them well, but I don't think the average HN user realizes what they could do with them, even for personal use. I think they're useful for all things of tracking, from to do lists to even recording time spend. Combine that with some simple scripts that work with csv, and you have a powerful setup with little programming.