Hamachi is clever and neat but ultimately falls down because of its proprietary heritage.  Windows users don't really care because they are used to closed software, however the Linux and Mac versions are woefully outdated and lack a complete feature set. Further more, Hamachi really doesnt give you much control, even if you pay for the premium service.

So I have scoured the interwebz in search of an open source equivalent. Surely some crafty people have begun an orchestrated cloud VPN style project which can spare us from the follies of Hamachi, whilst still providing a work around for miserable network code in games.

Enter Tinc.

"Tinc is a Virtual Private Network (VPN) daemon that uses tunnelling and encryption to create a secure private network between hosts on the Internet."

Tinc is very nice because it does exactly what Hamachi does, but allows you to run it on any platform (ie Windows, Windows 64bit, Linux on almost any architecture including WRT routers, OpenBSD, NetBSD, FreeBSD, Solaris, MacOSX/Darwin). It supports IPv4 and IPv6, and allows you to chose your own IP scheme. It automatically meshes in the same way hamachi does, so that the server doesnt require huge amounts of bandwidth or data. Supports static or DHCP assigned IP addresses and allow you to run your own server. So availability is in your hands!

You can check it out on wikipedia or their website