What we are using on IPv6 (2):

IPv6-specific stuff
Transport Relay Translator
node information queries
for address-to-hostname resolution
site-scope anycast
for DNS server discovery
privacy extension of stateless autoconf
6to4
we are not using it in our backbone, but we've developed it and we know there is a certain amount of users.
IPv6 links
Ethernet, ATM, serial line, tunnel, PPP