Some commercial products like hamachi (logmein) and 2x.com's stuff.
Also freenx and the other nx's.
Some chat software may work.
Two issues really. One is how to make a secure connection and the other is the remote desktop part. Some apps have both while others need two parts to work securely.
A web page with some review. http://www.techradar.com/news/softwa...clients-716346