In the second half of the 20^th century, people feared that nuclear war (especially nuclear winter) might snuff us all out. Later, with the arrival of subtle sensors and global meteorological studies, we worried about ozone holes and industrial pollution and an anthropogenic Greenhouse effect combining to blight the biosphere. Later still, the public became aware of the small but assured probability that our world will sooner or later be struck by a `dinosaur-killer' asteroid, which could arrive at any moment. For the longer term, we started to grasp the cosmic reality of the sun's mortality, and hence our planet's: solar dynamics will brighten the sun in the next half billion years, roasting the surface of our fair world and killing everything that still lives upon it. Beyond that, the universe as a whole will surely perish one way or another.
        Take a more optimistic view of things. Suppose we survive as a species, and maybe as individuals, at least for the medium term (forget the asteroids and Independence Day). That still doesn't mean there must be a Spike, at least in the next century or two. Perhaps artificial intelligence will be far more intractable than Hans Moravec and Ray Kurzweil and other enthusiasts proclaim. Perhaps molecular nanotechnology stalls at the level of micro-electronic machines (MEMS) that have a major impact but never approach the fecund cornucopia of a true molecular assembler (a `mint', or Anything Box). Perhaps matter compilers or replicators will get developed, but the security states of the world agree to suppress them, imprison or kill their inventors, prohibit their use at the cost of extreme penalties. Then we have option  

[A ii]    No Spike, steady as she goes

This obviously forks into a variety of alternative future histories, the two most apparent being

[A ii a]   Nothing much ever changes ever again

which is the day-to-day working assumption I suspect most of us default to, unless we force ourselves to think hard. It's that illusion of unaltered identity that preserves us sanely from year to year, decade to decade, allows us to retain our equilibrium in a lifetime of such smashing disruption that some people alive now went through the whole mind-wrenching transition from agricultural to industrial to knowledge/electronic societies. It's an illusion, and perhaps a comforting one, but I think we can be pretty sure the future is not going to stop happening just as we arrive in the 21^st century.
        The clearest alternative to that impossibility is

[A ii b]   Things change slowly (haven't they always?)

Well, no, they haven't. This option pretends to acknowledge a century of vast change, but insists that, even so, human nature itself has not changed. True, racism and homophobia are increasingly despised rather than mandatory. True, warfare is now widely deplored (at least in rich, complacent places) rather than extolled as honorable and glorious. Granted, people who drive fairly safe cars while chatting on the mobile phone live rather... strange... lives, by the standards of the horse-and-buggy era only a century behind us. Still, once everyone in the world is drawn into the global market, once peasants in India and villagers in Africa also have mobile phones and learn to use the Internet and buy from IKEA, things will... settle down. Nations overburdened by gasping population pressures will pass through the demographic transition, easily or cruelly, and we'll top out at around 10 billion humans living a modest but comfortable, ecologically sustainable existence for the rest of time (or until that big rock arrives).
        A bolder variant of this model is

[A iii]    Increasing computer power will lead to human-scale AI, and then stall.

But why should technology abruptly run out of puff in this fashion? Perhaps there is some technical barrier to improved miniaturisation, or connectivity, or dense, elegant coding (but experts argue that there will be ways around such road-blocks, and advanced research points to some possibilities: quantum computing, nanoscale processors). Still, natural selection has not managed to leap to a superintelligent variant of humankind in the last 100,000 years, so maybe there is some structural reason why brains top out at the Murasaki, Einstein or van Gogh level.
        So AI research might reach the low-hanging fruit, all the way to human equivalence, and then find it impossible (even with machine aid) to discern a path through murky search space to a higher level of mental complexity. Still, using the machines we already have will not leave our world unchanged. far from it. And even if this story has some likelihood, a grislier variant seems even more plausible.

[A iv]  Things go to hell, and if we don't die we'll wish we had

This isn't the nuclear winter scenario, or any other kind of doom by weapons of mass destruction--let alone grey nano goo, which by hypothesis never gets invented in this denuded future. Technology's benefits demand a toll from the planet's resource base, and our polluted environment. The rich nations, numerically in a minority, notoriously use more energy and materials than the rest, pour more crap into air and sea. That can change--must change, or we are all in bad trouble--but in the short term one can envisage a nightmare decade or two during which the Third World `catches up' with the wealthy consumers, burning cheap, hideously polluting soft coal, running the exhaust of a billion and more extra cars into the biosphere...
        Some Green activists mock `technical fixes' for these problems, but those seem to me our best last hope.[6] We are moving toward manufacturing and control systems very different from the wasteful, heavy-industrial, pollutive kind that helped drive up the world's surface temperature by 0.4 to 0.8 degrees Celsius in the 20^th century.[7]
    Pollsters have noted incredulously that people overwhelmingly state that their individual lives are quite contented and their prospects good, while agreeing that the nation or the world generally is heading for hell in a hand-basket. It's as if we've forgotten that the vice and brutality of television entertainments do not reflect the true state of the world, that it's almost the reverse: we revel in such violent cartoons because, for almost all of us, our lives are comparatively placid, safe and measured. If you doubt this, go and live for a while in medieval Paris, or palaeolithic Egypt (you're not allowed to be a noble).