Port forwarding for RDP to a box behind a firewall
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ssh -L 3389:<dest host>:3389 username@ext.host.example.com
rdesktop localhost

Port forwarding for VNC to a box behind a firewall
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ssh -L 590x:<dest host>:590x username@ext.host.example.com
xvncviewer localhost:x
  where x is the vnc session number you want