I've been using Hamachi successfully for quite a while. LogMeIn seems to have bought them and it seems to remain a separate product (that also has free and paid versions).

Hamachi sets up a point-to-point network between individual computers. You set it up and configure which computers you want networked together and will then be able to use virtually any network services between them. In particular, you can use Remote Desktop or VNC. Hamachi runs under Windows 2000, XP, Server 203 and Vista, Linux, and OS X.