Open Source Tax Software
Tax software lives or dies on one thing: whether it correctly encodes the law for your jurisdiction and year, which is why this is a thin field rather than a crowded one. Most open source options here are calculation and microsimulation engines - they model the rules you can read and check, rather than full consumer filing apps - so you can verify the math and keep your income data off a third party's servers.
TaxHacker
Self-hosted AI accounting for receipts, invoices, and transactions with custom prompts and categories
IRS Direct File
Source code for the IRS service that files federal tax returns through a plain-language interview
UsTaxes
Open-source web app for filing US federal Form 1040 with browser-side tax calculations
OpenFisca-France
OpenFisca model of the French tax and benefit system for microsimulation and rules-as-code
OpenFisca Core
Python microsimulation engine for modeling tax and benefit legislation as code
PolicyEngine US
Microsimulation model of the US state and federal tax and benefit system
HabuTax
Open-source US personal income tax solver with command-line output and PDF form filling