If you have a dynamic IP address at home, you can use a dynamic DNS service such as dyndns.org, plus a client that runs on your Mac, so you can do something like
ssh -l <yourloginid> <yourname>.homeip.net
for example
It's often the case that the ssh port is open (port 22, IIRC) by default - if not, you will have to set up port forwarding on port 80.