Remember my post about IPCop? Well, here's yet another gem that's free for those who are willing to work for it - OpenVPN. The reward is an easy to set up, easy to administer Virtual Private Network connection. The OpenVPN GUI provides an easy way for clients to connect. http://openvpn.se/ also explains how to create an easy-to-install client package that can customized for your own VPN (Not for NEWBIES!).
Before you freak out and try putting this all together, check out this tutorial on getting OpenVPN to work with your IPCop. The Zerina VPN Plugin is beautifully made, thought not fully implemented, and makes ready-to-use .ovpn configuration files. It also makes and exports its own certificates, and lastly, and possibly the biggest headache it solves, it automatically configures client and server routes.
So now that you can remotely join your internal network, what will you do with it? 1st up, Windows Network browsing services won't work. If you have a DNS or WINS server on your internal network, point your TAP Interface's at one or both of these, and you'll be able to use your familier Network Neighborhood stuff. If these services are unavailable, then you're stuck using hosts IP addresses (ie \\192.168.0.20).
Beyond Windows File Sharing, most everything else should work through your shiny new Ecnrypted Tunnel without a hitch.