jq — CLI JSON processor written on С. Allows to extract data from JSON, modify it, supports math, variables, regexes, even has a lot of built-in functions. Installable from Debian repositories.
jqplay.org — online playground for jq. Great to try it, debug expressions, have a handy cheat sheet.
gjson — must-have Go library to get values from a complex deeply nested JSON. Has a simple and intuitive syntax.
jj — CLI tool written on Go, wrapper around gjson. Hence less powerful than jq but much simpler and more friendly.