One method I've used for accessing my home box from the interweb is to use a dynamic DNS account. I have a script that is run from cron that does a lookup on a certain hostname and updates an iptables chain whenever the IP changes.
I don't need to leave SSH open, I just carry the username/password for the ddns account and update it whenever I need access from somewhere. Within a few minutes a hole is opened through the firewall and I can log in.
