Enabling IPv6 on OS X. Very neat. Very easy. Dancing turtles everywhere. Use en1 instead of en0 to use the AirPort interface instead of ethernet.
Of course, as Aaron points out, the IPv6 transition is a big mess, so who really cares? I've been vaguely contemplating to make my private network run IPv6 instead of on 10.x addresses, but the only advantage is the neat factor (and that would grow old fast I'm sure) and there are many disadvantages. Hmn... Go and read djb's analysis of the IPv6 transition. I just read it a few days ago from a reference on the qmail or the djbdns list, funny that it came up again here.