It was on my list of things to check out for quite a while now, and now I wish I had tried it out much sooner. Seriously, if you ever want to connect to remote computers, be it your home system while you're away or to your friend's computer, check this program out. Zero-configuration VPN make me happy. So what would you really use this for?
Well...
Since it's zero configuration, it doesn't matter if both computers you want to virtually-network together are behind firewalls with no ports open or even if their IP's change. So the first use that came to mind was being able to close any open ports you might have port-forwarded through your firewall-router that you use for remote access. RDP can be closed. No need for FTP. If you previously were using SSH, no need for port 22! People already using a VPN already have these benefits, but this is just plain easier.
So that's nice, but what would an average person use this program for? Ok, how about games which might only work on a LAN? You can create groups of computers, and your computer can be a member of multiple groups. You could have a gaming group of friends and can play a private game of Starcraft or maybe a racing game. Whatever.
It also can be nice for sharing files with windows file sharing. Heck you could print something out at home if you had the printer shared.
I think all of these things have been stated elsewhere. So here is something I haven't read elsewhere yet. You could use it to share iTunes music! It's perfecty legal since it's just like your computer is on the local network with the iTunes machine. No songs are downloaded and saved - just streamed. So it would work great for listening to your home computer's music collection at work, but if you take it a step further you could have groups of friends all sharing their libraries with eachother. Of course as Chris reminded me, you can't listen to protected songs unless your computer is authorized but all those CD's you own and imported to iTunes will work great :)
What about video? People using sageTV and other networkable PVR apps could connect to eachother. Or you could watch your recorded shows or live tv from an internet cafe. If nothing else you could just share the video directories (read only of course) so that there is no worry of someone accidentally messing with your recordings or schedule.
On the geekier side of things you could connect all your virtual machines with Hamachi. That would sure simplify some things. Have that test system set up to show your latest work but no easy way to access it at a client's location? Well that's just simple now.
I've read of people maybe having an easier time networking xbox and other consoles together easier. Quite cool.
It's not all peaches though. With VPN made so much simpler, people will have to be careful exactly who they hook up with. You are essentially letting someone into your LAN, though relaying to other local machines which aren't on Hamachi isn't supported right now at least (which techincally differentiates it from a normal VPN which connects a client computer to the network rather than just connecting two computers). Everyone who might be lax on their shared directories will have to be careful - increase security or remove shares of important things.
Hamachi is available for Windows, Linux, and soon the Mac. It is free but there will be a premium version with extra features available at reasonable subscription pricing
If you want more info then here are screenshots and if you're more audio-oriented you can hear Steve Gibson talk about it or Leo Notenboom. Also if you have any trouble with Hamachi or getting somethign to work with it, don't forget to check out the Hamachi forums.
A final note - I did run into a slight hiccup when running through the introduction wizard because the "test" group it told me to join was full and it wouldn't let me continue until I joined it. Eventually space became available, but if you're impatient you could just close the wizard if the group is full because I don't remember there being anything too important after that point.