Hamachi will allow you to establish secure direct connections to PCs that are not accessible otherwise due to the restrictions in network setup. By installing it on two computers and clicking a couple of buttons you can connect them instantly and securely.
Hamachi is a networking technology enabling any two computers on the Internet to talk directly to each other regardless of the presence of firewalls or address translation devices on the route between them.
Most interestingly, it enables peer-to-peer communications between two computers residing behind two different connection sharing devices. In geek language - it allows for bidirectional NAT traversal.
As of January 21, 2014, LogMeIn Free (Hamachi) is no longer available. Users who will be impacted by this change will receive notification from the author by email and via an in-product message.