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Infrastructure that powers financial technology by enabling applications to connect with users' bank accounts.
Simulator for personal finance to plan for FI & other goals
Wow this is like a flashback to my own experiments in Excel, prior to going full mad-scientist for the past 6 months and creating a web app for FI/FIRE planning called ProjectiFi: https://projectifi.io/ I had been using similar functions in parts of my spreadsheet, but as I kept adding more and more nuance and flexibility to the life planning and projection aspects, eventually the formulas just reached such a...
A personal and small-business financial-accounting software, licensed under GNU/GPL and available for Linux, Windows, Mac OS X, BSD, and Solaris.
For a non-startup personal open source project, you can consolidate all that data in the desktop program https://gnucash.org/ . Offhand, I suspect you could layer all those features you mention, using a mix of GnuCash customizable existing reports, and their (poorly documented) extension in Guile. The UX might not be what you want with just reports and scripting, but you can definitely analyze data and generate...
A personal budgeting tool with multi-currency support
I have been using Lunch Money after it was mentioned here and am pretty happy with it. It took a while for the API to launch but it's in beta at the moment and they're pretty responsive to user feedback.  https://lunchmoney.app.