As I've previously logged my Fritz!Box 7390 on O2 home broadband settings, I figured it might be a good idea to also chuck my IPv6 setup here as well, as the inevitable day when I break my router/modem/switch/magic box and need to factory reset it must be getting closer ;-)
To setup the IPv6 tunnel, first, get an account with tunnelbroker.net, and set up a tunnel there. I'm not going to cover that, if you can't figure it out, you probably don't need to get IPv6 set up ;-P
On the Fritz!Box, go to Internet -> Account Information -> IPv6
Tick "IPv6 support enabled", and select the "Always use a tunnel protocol for the IPv6 connection" radio button.
Select tunnel protocol "6in4", then fill in the boxes from the information on the tunnelbroker.net page. The IPv6 address for the tunnel endpoint will probably end with a :1, and what I did for the "Local IPv6 address:" was make it :2 - logical enough, right?
I didn't need to set a manual MTU, and I made sure that "Also announce DNS server via router advertisement (RFC 5006)" was ticked.
That's about it.
And if you're wondering why the fuss and "hassle" of setting up IPv6? Well, you try playing loopsofzen.co.uk without it (unless it's been enabled on IPv4 as well, in which case I look silly now).
Oh, and don't forget the usual disclaimer about your varying mileage, this might not work for you, I based it on my setup, it works for me, I didn't test it anywhere else, etc, etc, etc.