Given a Linux machine with IPv4 connectivity but no native IPv6 connectivity, what is the easiest way to setup free IPv6 tunneling?
Which tunneling providers are recommended and which steps are needed in the setup?
http://tunnelbroker.net/ seems like the most commonly-recommended option; documentation for setting it up is available on that site. Personally, in a perfect world I'd prefer 6to4 with it's anycasty goodness, but there are some interesting security concerns there. I've never setup any of them, myself, since my ISP provides native IPv6 connectivity over my DSL connection (w00t for forward-looking ISPs).
I used https://www.sixxs.net/
But whichever tunnel broker you use, you will have to follow their instructions since there are several ways of doing it. It's pretty easy, in general.
The other consideration is security, because the security properties of v6 are quite different... and now every host on your network can potentially have a global address, you have to think about incoming connections quite carefully.
Most Linux distributions don't ship with a v6 firewall configuration package yet. Very recent versions of Shorewall do have good v6 support, however, but you may have to install that from source (not scary, it depends only on perl).