Is there a free linux (Ubuntu and/or RHEL) vncserver which works with IPv6?  From what I can see, the normal VNC RPMs contain the Free Edition of RealVNC which does not have IPv6 support.  The Enterprise Edition looks like it supports IPv6, but that costs $50 a server.  RealVNC is also the only program that I could find which supported IPv6 as a Windows client, and that's also $50.00, unfortunately, I can't afford both client and server, especially since I have multiple IPv6 servers that I want to interact with.

There was a TightVNC port with IPv6 (http://jungla.dit.upm.es/~acosta/paginas/vncIPv6.html), but that web page is dead.

TigerVNC does not support it.
The standard "vnc4server" and "xvnc4viewer" in Debian (and I think in Ubuntu as well) do support IPv6.
RealVNC Enterprise Edition Viewer for Windows is zero-price, and supports IPv6 as well.

IPv6 is supported in the Viewer since 09 Sep 2009, in the Server since 19 Nov 2009.
http://packages.debian.org/changelogs/pool/main/v/vnc4/vnc4_4.1.1+X4.3.0-37/changelog