PROJ bindings are available for a number of different development platforms.
proj.xll: Excel add-in for PROJ map projections
Fortran-Proj: Bindings for PROJ in Fortran (By João Macedo @likeno)
go-proj: Go bindings for PROJ.
PROJ-JNI: Java Native Interface for PROJ
Proj.jl: Julia bindings and idiomatic wrapper for PROJ.
fProj4: Bindings for PROJ in MySQL
Geo::LibProj::FFI: Foreign function interface for PROJ (version 6 and newer) in Perl
pyproj: Python interface (wrapper for PROJ)
proj4rb: Bindings for PROJ in ruby
proj: Rust bindings for the latest stable version of PROJ
proj4tcl: Bindings for PROJ in tcl (critcl source)
PROJ VB Wrappers: By Eric G. Miller.