As a Graphic Designer, I'm at a computer all day doing work. Though, the majority of time is spent in InDesign, and then Illustrator, PhotoShop, and of course Outlook (ugh). I've got very little use for the typical Office Suite software, and at home I use (in addition to the above Adobe programs) a combination of Google everything, and just Text Edit to take care of business. I've recently been utilizing the spreadsheets from GoogleDrive (formerly GoogleDocs, which is still how I refer to it in my head) like crazy.
Once sales from my Etsy shop started taking off, I moved from doing math on paper to starting a spreadsheet with formulas to keep track of orders and costs as well as timelines. This has seriously been a blessing for me, as I've well surpassed the 100 stamps mark. I don't know how I could have kept track of it without the spreadsheet. I've also got it set up to keep track of margin percentage as well as running totals, which are great motivators for me to keep investing time in to my stamps. I may have gotten somewhat carried away with the color coding too.
Perhaps little known fact, I'm a co-coordinator for the SPARK Mentorship program through the Upstate NY Chapter of AIGA. I wrangled the applications coming in, and yes, there was a big 'ol spreadsheet for that too, very very color coded. Being a visual person, I've found having some sort of obvious clue to the grouping of information (such as color) can make much easier to make sense of mountains of data.
Even further, I'm using spreadsheets to keep track of my resolutions/goals for 2013 as a whole, and then a few more specifically too (such as miles walked).
Strange, but even as recently as a year ago, I've have never thought I'd be spending significant time punching numbers in to a spreadsheet--that's what other people do. Other people in boring, soul-sucking jobs. What skills that you learned in high school do you find have come in handy more than you'd ever have imagined?