GUI: Racket Graphics Toolkit describes the Racket graphics toolbox, whose core is implemented by the gracket executable.
FFI: Racket Foreign Interface describes tools for using Racket to access libraries that are normally used by C programs.
Web: Racket Web Applications describes the Racket web server, which supports servlets implemented in Racket.
Racket Documentation lists documentation for many other installed libraries. Run raco docs to find documentation for libraries that are installed on your system and specific to your user account.
PLaneT offers even more downloadable packages contributed by Racketeers.