A Chart That Reveals How Science Fiction Futures Changed Over Time. Often roughly level, with big spikes for near-future during times of rapid scientific and technological change (it says here) and what really makes me wonder is why the far future almost vanished in the 90s except for Futurama... and that Star Trek gets a whole point of the future to itself.
The comments below, in a change from my usual "I read the comments..." wail of existential despair, are also quite interesting, about how different SF ideas come in and out of favour. When did you last see a serious SF version of telepathy, for example? Hmm, well, how serious is serious? Trinity is serious SF here and space opera there and post-apocalypse horror over yonder, does it count?