OpenVPN
OpenVPN needs an OS X and Windows front-end for client systems and a generally non-sucky configuration interface for systems that are going to act as gateways for road-warrior users. I've gotta think there are a lot of people in the same boat as me who really don't want to be hacking their ifup scripts to change their routes every time your laptop moves to a new network.
I bet you could even charge for code like that.