So the most popular open-source Postman alternatives seem to be

HoppScotch (47.7k stars, 2019, Vue): https://github.com/hoppscotch/hoppscotch

Insomnia (24.2k stars, 2017, React): https://github.com/Kong/insomnia

Restfox (405 stars, 2022, Vue): https://github.com/flawiddsouza/Restfox

bruno (30 stars, 2022, Tauri, Electron, Next): https://github.com/usebruno/bruno

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You can find more here: https://alternativeto.net/software/postman/