LogMeIn Hamachi

LogMeIn Hamachi is a VPN service that easily sets up in 10 minutes, and enables secure remote access to your business network, anywhere there's an Internet connection.

It works with your existing firewall, and requires no additional configuration. Hamachi is the first networking application to deliver an unprecedented level of direct peer-to-peer connectivity. It is simple, secure, and cost-effective.

Link:
https://secure.logmein.com/products/hamachi/vpn.asp

Difference between the free and premium version:
https://secure.logmein.com/products/hamachi/advantages.asp