What future technologies does particle physics and string theory promise? What practical application can we expect from particle physics a century or two from now? What use can we make of quark-gluon plasmas or strange quarks? How can we harness W- and Z-bosons or the Higgs boson? Nuclear physics has given us nuclear plants and the promise of fusion power in the near future. What about particle physics? If we extend our timeframe, what promise does string theory give us? Can we make use of black holes?
 A: The experimental testing of these theories, and further assuming we might be able to test string theory sometime in the next decades, will result in new methods of detection, new techniques in data management and filtering signal from noise, and push other developments.  These may then be transferable to other applications.  The Apollo lunar program had a similar effect, though we brought nothing back from the moon which had any great utility.
A: You asked for speculation, so here is outlandish speculation:
We can do engineering all the way down to the atomic scale. The reason is that life is made of atoms, and biology is complicated processes capable of general purpose computation, so atoms can do complicated things. There is no reason that we should be able to do engineering with nuclei, because our universe doesn't embed life in nuclear structures.
But that doesn't mean that you can't try. Suppose you could build a pion laser, and stabilize it by the appropriate methods, by surrounding it with the appropriate reflectors to prevent 2-photon decay, or change to charged pion very quickly to prevent decay, or ... I don't know, or else this wouldn't be speculation. Then you would be able to do nuclear engineering, meaning move nuclei around like you move atoms with light lasers, and pump energy into hadronic states at will, not by collisions, but by engineering. Without good bosonic fields which you can manipulate over relatively large scales, there is no hope for any of this.
Using a pion laser, you might be able to blow up elementary particles in a spherically symmetric way, like balloons, and let them collapse onto themselves to make black holes. This requires a controlled implosion, because the mass of a nuclear scale black hole is that of a mountain. If you can somehow make a symmetric implosion which shrinks the pumped up hadron by 10 orders of magnitude, to get some black hole states at some heavy but achievable mass, you can make monopoles, black holes, strings, and turn anything into energy.
I spent many hours of my youth trying to make this hopelessly futuristic idea work, because it is the only hope we have of investigating the Planck scale by even speculatively achievable technology. I think this type of thing might work out in 200 years, and produce a monopole-driven total mass-energy converter.
A: Allow me to answer your question with some quotes:


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*"The energy produced by the atom is a very poor kind of thing. Anyone who expects a source of power from the transformation of these atoms is talking moonshine." —Ernst Rutherford

*"There is not the slightest indication that nuclear energy will ever be obtainable. It would mean the atom would have to be shattered at will." —Albert Einstein

*"There is no likelihood that man can ever tap the power of the atom. The glib supposition of utilizing atomic energy when our coal has run out is a completely unscientific Utopian dream, a childish bug-a-boo." —Robert Millikan

*"Radio has no future." —Lord Kelvin

*"The more important fundamental laws and facts of physical science have all been discovered, and these are now so firmly established that the possibility of their ever being supplanted in consequence of new discoveries is exceedingly remote.... Our future discoveries must be looked for in the sixth place of decimals." —Albert. A. Michelson, 1894

*"There is nothing new to be discovered in physics now. All that remains is more and more precise measurement." —Lord Kelvin, 1900
Also, I couldn't find a good quote for this one, but it was widely believed that number theory was an abstract mathematical discipline with no practical application. Now it is the basis of all modern cryptography.
Basically the idea I'm trying to get across is that it's impossible to predict the future applications of basic research.
A: Quantum Chromodynamics, the electroweak theory, or general theory of relativity - or quantum gravity and string theory - are not methods to obtain new devices; they're theories meant to understand the truth about the Universe.
I find it unlikely that any of those things will become practically useful. It may still hypothetically happen, but if it will, no one can predict how this could happen at present - but even if those things happen to have practical applications sometime in the future, that's not why they've been and why they are being studied.
A: Cultural enrichment.
Let me explain. String theory is a work of art. Art has no practical application, you say? No! It is practical in enriching culture and uplifting the emotions of mankind. So is string theory. The emotional satisfaction that you get out of it. Shout it out to the whole world!
We fund art, so why not string theory! Oh, yeah!
A: Here are some possible applications of neutrinos
Submarine neutrino communication
Demonstration of Communication using Neutrinos
Searching for cavities of various densities in the Earth's crust with a low-energy electron-antineutrino beta-beam
A: I feel quite sure that we will have technological breakthroughs but i think the most important findings will successfully link perception/awareness/consciousness with the so called external universe.
A: There are applications of particle theory. Well, it depends on what you call particle theory. There are projects of sensors which use neutrons to detect explosives (google gave me e.g. this paper). To develop such a device excellent understanding of particle physics is needed. Neutrino telescopes will be perfect devices to study the Sun and the Earth core soon. Which is useful for weather and cataclysms prediction. You might think that these applications are too engineering, but you never know which tiny details you need, so you have to discover all of them. 
For astrophysics one of interesting applications I've heard of is using pulsars as a positioning system. Instead of expensive and short-living sattelites which work only at the Earth you may use perfectly stable coordinate system which is Ok for few light years off the home planet. It is not string theory, but just an example of engineering stuff which comes out of pure science in an extremely unexpected way.
Speaking of science fiction, which you probably had in mind. String theory studies time and space. Right now we may operate (or at least know how to operate) on matter (all conventional technoloies, nuclar stuff, etc), fields (radio) and combination of the two (lasers and similar). The thing which is impossible to control is gravitation. General relativity explains why: gravitation is tightly connected with space and time itself. However, it does not explain all the details. To know the details we will need string theory or its successor/competitor. XX century taught us that you can be never sure that you know everything (see citations from Lord Kelvin). Small but annoying problem of inconsistency between general relativity and quantum mechanics might produce as many new physics as "small problem" with atomic spectra. Or it might not. You never know. 
