The Hurricane Electric IPv6 Tunnel Broker service enables you to reach the IPv6 Internet by tunneling over existing IPv4 connections from your IPv6 enabled host or router to one of their IPv6 routers. To use this service you need to have an IPv6 capable host (IPv6 support is available for most platforms) or router which also has IPv4 (existing Internet) connectivity. their  tunnel service is oriented towards developers and experimenters that want a stable tunnel platform.

More info: http://www.tunnelbroker.net/