Ceefax is certainly something that is perhaps more powerfully of its era than almost anything else.
When I was really little ceefax was just kinda broadcast on shit channels when nothing was on, but later on, when I realised that it was (or maybe it simply became) possible to turn it on and interact with it, that was really exciting.
To tie in with what I was saying in Sufi's thread about blunted cutting edge, I remember visiting my Grandparents in Manchester and me and my uncles checking the score in the United game using Ceefax - in (as I remember it at least) almost the last minute, Man U got a win cos of a goal by an unknown sub whose name was mangled by the software - we were all saying "Who the fuck is Lion Riggs" - both obsolete now of course.