I've been doing some playing with IPv6 lately, since the world is out of IPv4 addresses (wait, didn't that happen a long time ago??? Whatever...). It's been really interesting to learn about the actual protocol and how it works compared to IPv4. I've got a fully routed /64 IPv6 network at home, but just a couple systems at home using auto-configured IPv6 address. I've thoroughly enjoyed it, especially because I've been putting off learning about it for so long (truth be told, I was a bit intimidated by it -- didn't need to be).
If you want to play with IPv6 and your ISP doesn't yet support native IPv6, you can easily set up a tunnel through http://www.tunnelbroker.net/
(provided by Hurricane Electric Internet Services) for free. They also have free DNS hosting (with full support for AAAA
and reverse IPv6 records, of course) and they offer a free, online IPv6
certification (I got to the "Sage" level last week).
I found IPv6 Essentials
from O'Reilly to be very helpful in understanding how it all comes
together. You can get it in a DRM-free ebook format from O'Reilly's site, which is pretty awesome.