Er...
Nuclear energy is the only viable "alternative" energy source we have. Period.
Do the research. Other options are simply either a net energy loss at this time (ethanol, biodiesel), are too intermitent and present storage issues (solar, wind), simply not enough to supply demand and may cause other environmental issues (hydro), or the tech simply isn't quite up there yet (hydrogen as a means to convert electric (nuclear) power to a transportable fuel source).
I'm not saying considerable effort and research should not go into the alternatives - obviously we need to figure out a way to get off fossil fuel dependence ASAP. However, any near-term solution in the next couple decades will have to have a very large part of it being nuclear based, or some huge leap in tech will have to happen to make something else viable.
Right now, things like Biodiesel work because people are throwing away old fuel sources (vegetable oil), and demand is simply not high. More petroleum goes into growing said plants that make the oil, then you actually get out. Same goes for ethanol derived from corn (although it's starting to become promising, which is cool) - it's debateable about ethanol derived from sugarcane however.
What I see happening "when the oil finally runs out" is actually a pretty natural transition. As oil becomes more and more expensive, you will see the "hippies" being overruled on blocking all other forms of energy. Nuclear is the only viable stop-gap (aside from coal, but I really hope we don't start burning more of that...), until "something better" makes sense. In the future, I more or less see energy being provided by "the grid" - e.g. nuclear, solar, hyrdo, whatever and then for non-electric means being converted into some transportable and high-density form of energy. For example, nuclear plants dedicated towards simply making hydrogen for cars. Or dedicated simply to distilling corn to create ethanol, etc. This is sort of starting to happen to the electric grid - for the first time since.. well, before I was born, there have been new nuclear power plants approved. This to me is a good sign in the right direction, but we'll see how it goes.
While I don't think the transition will be all too easy, I also don't see the world ending like folks say. Oil will gradually rise in price, baring any super wars and such, and as it does, alternatives will start to make economic sense and you will see investment poured into them. You already are seeing this on a small scale, and as oil continues to rise it will grow exponentially.
Of course, we could all face complete economic collapse and be totally fucked. Who the hell am I to predict the future?
-Phil