[surki]

  Got a computer behind a firewall whose configuration 
  you don’t have access to? It’s pretty easy to get the 
  computer behind the firewall to poke out to another 
  server.

  (step 1, from the computer you wish to access)
  derwiki@firewalledcomputer:~$ ssh -R   localhost:2002:localhost:22 mypublicserver.com

  (step 2, from any computer than can access  
   mypublicserver.com)
  derwiki@mylaptopontheinternet:~$ ssh mypublicserver.com -p 2002
  (authenticate)
  derwiki@firewalledcomputer:~$

If you want to keep it running always, you may want to consider "autossh" (restarts ssh connections if they ever exit/disconnect)