You have a couple of options, depending on your configuration.
I assume you have a router in your intranet that connects your home computers to your ISP (cable modem, dsl, T1)? You will want to go into the settings of your router, which you can usually do by using a web browser to go to 192.168.0.1 (if your network is 192.168.0.*, or the equivalent).
Once in your router, you will setup "Port Forwarding". Setup port 80 to point to the intranet IP address of your Mac.
You will then need the public IP of your house. The easy way to get this it to browse to
http://whatismyip.com. You can then put this IP Address into any web-browser on the internet and it will go to your house, the router will then route the HTTP request (on port 80) to your mac, which will serve the webpage.
Alternatively (and this is what I do), get an account at dyndns.org. This allows you to have a fake hostname that will point to your dynamic IP address. Therefore, you can register for something like: captcurrent.home.org and that will route to your mac.
Let me know if you need any clarification on the issue.
Addendum: you can do the similar thing by port forwarding remote applescript (port 3031) to your mac in addition to the HTTP port 80. This would then let you use IndigoWidget over the internet - though it may be slow.
I've posted these instructions on the IndigoWeb homepage:
http://highearthorbit.com/software/indigoweb