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How do I connect two machines that are both behind seperate firewalls

Here is my situation, I want to establish a connection between two machines via ssh, however both the target and destination machines are behind firewalls, neither of which I have any control over. I do however have complete control over both machines.

It is my understanding that if one side of the connection had a public address I would be able to initiate a connection from either the target or destination using some combination of ssh port forwarding and/or http tunnelling to get through the one firewall. However with firewalls on both sides neither side can initiate a connection since all incoming connections will be blocked.

I believe I need some sort of intermediary through which both machines can establish an outgoing connection and that will bridge the connection between both machines.

My first question is are there any free or relatively cheap services out there that will act as the intermediary? There seems to be tons of free dynamic DNS and web proxy services out there but I don't think these will be of any help to me.

Failing that my other option is to use CGI scripts hosted somewhere on my own public website that will act as the intermediary. Are there any CGI scripts out there that do this or something similar to this already? If I was going to write something from scratch in Perl, is it simply a matter of waiting for both ends to connect and than just passing the packets between them, with maybe some specified time-out period with no activity? The Net::HTTPTunnel CPAN modules looks promising.

Thanks,

-- Darrel Yurychuk