GUI: PLT Graphics Toolkit describes the PLT Scheme graphics toolbox, whose core is implemented by the mred executable.
FFI: PLT Scheme Foreign Interface describes tools for using Scheme to access libraries that are normally used by C programs.
Web Server: PLT HTTP Server describes the PLT Scheme web server, which supports servlets implemented in Scheme.
PLT Scheme Documentation lists documentation for many other installed libraries. Run plt-help 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 PLT Scheme users.