UltraVNC has a utility that can put together a single executable that you can send to someone and when they run it, it will establish a vnc connection with your computer. It requires no installation and I think it works for limited users and will work if they are behind a firewall.
Otherwise, they could install UltraVNC on their computer, open tcp port 5900 on their local software firewall, forward port 5900 from their home router/firewall and then you can initiate a session from the UltraVNC Viewer on your computer.