More to be filled later...suggestions are welcome!
finger, nslookup, ping, traceroute, whois: UK IPv6 Resource Centre / The test page
ping, traceroute, tracepath, 6bone registry, DNS: JOIN / Testtools (German language only, but should be no problem for non German speakers)
traceroute6, whois: IPng.nl
List of worldwide all IPv6-aggregated IP-Blocks (daily update) same service from Ripe