We at BlinkenSisters.org are planing to support IPv6 before the end of the year (BS uses TCP to download addons in-game). Currently, the biggest obstacle is the lack of IPv6 support in SDL_Net. I've some an old patches for the library. As it seems the maintainer of the library isn't really interested in releasing an IPv6 enabled version (changes the API), i might go the extra mile and do a fork myself. Well, so long, beloved spare time... Of course, when we get the first IPv6 enabled version of BS (and SDL_net) to run, we'll start to need Beta-Testers... BTW, you won't have to "rush out and buy" BlinkenSisters. It's free...

Our GPL game PokerTH (http://pokerth.net) has IPv6 support :-). It is available for Windows, Linux, MacOS and I think a BSD port is also available. You can also play on the internet server through IPv6.

There is also Quake 3, in the form of IO Quake 3: http://ioquake3.org/2008/04/21/ioquake3-now-ipv6-capable/ Quake 3 is mentioned on the "Sixxs Cool IPv6 Stuff" page, but the link ends up going to the main page of Hexago. Note you will need the commercial Quake 3 release for some of the data files.