Malbolge
Malbolge is an esoteric programming language designed to be extremely difficult to write and understand. It is mainly used for code obfuscation and as a programming challenge.
Hello, world code example:
('&%:9]!~}|z2Vxwv-,POqponl$Hjig%eB@@>}=
Popularity: Less than 1% of developers are using or have used this language.*
*According to StackOverflow's 2023 survey.