For reasons beyond my understanding, the user 2013 committee didn’t register a domain name for the website, and the official address of the conference is: http://18.104.22.168/congresos/useR-2013/. Not only is this impossible to remember for humans, but it won’t show up in search engines. So I decided to help them out and invest 8 euro to register a domain and point it to their server.
Hence, the user 2013 conference website can now also be reached through http://user2013.org. Note that this is not a mirror with different content or anything; the domain sends you straight to the official webpage. Once some other pages link to this URL, it should show up in Google soon enough so that useRs that want to register can find the site. I’ll try to update the domain if the official url changes again.