I've been building an app to replace a long-suffering Excel sheet I've used for retirement planning. Started off as a calculator, eventually turned into something like ProjectionLab, though with a focus on what-if scenarios (e.g., how many years can I knock off my working life if I downsize to a smaller house or relocate to another state?) and risk evaluation specifically for early retirement. Not super fancy, but it's already flagged some risks that my spreadsheet didn't, and it's fun to have a codebase that's fully owned/controlled by me.