IPv6 documentation prefix and IPv6 site/host list (posted 2004-01-04)
There is now an official IPv6 prefix set aside for documentation purposes: 2001:0DB8::/32. (Leading zero courtesy of APNIC.)
The how and why is documented at a page at APNIC. Note that there is also a prefix set aside for documentation purposes in IPv4: 192.0.2.0/24. See RFC 3330 for more information and other special IPv4 prefixes.
At prik.net there is now a list of IPv6-enabled hosts or sites. I have no idea how complete the list is, but it has more than 3000 entries so it's better than the manually maintained stuff in some other places. If the link doesn't work, this is probably because your browser doesn't understand compressed content. In that case, use the uncompressed version. The compression ratio is about 1 : 6.