Over the years, I've worked extensively with Excel and VBA.
For instance, I automated a monthly service report for my boss. He used to spend about one week each month, manually updating all pivot tables, checking the data and setting the indicators. His workload went down to a couple of hours each month.
Years later, I took over maintenance of some in house project planning tool (which was a mess). After some months, I decided to build a new tool. A colleague beautified it. It was well received.
As freelance, I'm developing a life planning application for a Dutch client. The application has tax calculations, mortgage calculations and pension calculations...