United Kingdom
The UK combines serious policy modeling with serious personal tooling: PolicyEngine-UK simulates reforms to the tax and benefit system, while crypto and capital-gains calculators implement HMRC's share-matching rules (same-day, bed-and-breakfast) that generic tools get wrong.
Open Making Tax Digital bridging software — MTD mandates digital filing through recognized software, and the recognition process has kept open source out.
4 projects tagged United Kingdom, ranked by community stars.
Crypto-currency tax calculator with support for all popular wallets, exchanges, explorers and migration from other crypto tax platforms
Calculate UK capital gains tax from broker exports — supports Charles Schwab, Trading 212, Morgan Stanley, Sharesight, Vanguard, and Freetrade.
An open-source static tax-benefit microsimulation model for the UK.
Beancount plugin for generating informational UK tax report (calculate capital gains tax)