There are several multiplayer games that you can play on SDF via the shell and clients run on your computer

• freeciv.sdf.org port 5555
• clients and info: Freeciv Wiki

• mud.sdf.org port 4000
• Clients on SDF:
  • 'mud' on the shell
  • TinyFugue: type 'tf mud.sdf.org 4000'
• Other Clients:
  • Windows: MUSHclient
  • Windows (non-free): CMUD
  • MacOS: Atlantis MU

• server: tetrinet.sdf.org
• clients: http://en.tetrinet.no/tetrinet-clients-download-1.php, tetrinet at the shell
• stats: http://tetrinet.sdf.org

• sdf.org port 5154
• clients and info: http://bzflag.org/