6to4: Hurricane Electric
It turned out to be extremely easy to register for a tunnel endpoint with Hurricane Electric's free registration form. My prefix was assigned in minutes and HE provided excellent instructions on how to configure my end of the tunnel.
(Unfortunately, my Linksys WRT54G router will not forward IPv4 protocol #41 traffic (IPv6 in IPv4) without free firmware like DD-WRT.)
Update: 6to4 via Hurricane Electric seems to work really well after flashing #OpenWRT onto the router!