Debian :: Remote Admin Of Computer Inside A Firewall

I have Linux computers that are often on client networks inside firewalls . They have access to the internet but I can't access them directly. I don't want to have to get the client to mess with NATing , etc . I have written a system that uses XMPP to allow me to send commands to the computers and run simple console commands which often time is enough. These are things like run a report and ftp up to server, restart a process, give me a process list. It works OK, but some networks even block this (Googletalk is not universally loved for some reason)

I am wondering if there is a prebuilt system that allows me a computer inside one firewall (say a standard adsl modem created one) communicate in some way with a computer inside a network created by another adsl modem. I could see some sort of console like logmein or pc anywhere but as much as I have looked I can't find anything. I have considered writing a cheap and cheerful system in http or even a TCP system using asyncore in python but there must be something existent. I have a server on the web that could be used as an intermediary, relay type thing. Basically some sort of chat server for consoles is what I am thinking about. Is there some sort of SSH voodoo that I could bridge/tunnel/vpn through.