[skrowl]

https://insomnia.rest/ is a pretty great alternative. The only thing you don't get for free is data syncing.

https://github.com/frigus02/RESTer is a Firefox (or Chrome) extension that does most of the same stuff that PostMan does without requiring a standalone client install.

[TAForObvReasons]

https://postwoman.io/ is also great for simple use cases (and is MIT licensed https://github.com/liyasthomas/postwoman)

[miket]

Seconded on Insomnia - I switched to it when encountering a bug in the encoding of the curl command generation in Postman, and haven't looked back.