The word "consciousness" is used in a variety of different ways, and there are large disagreements about the reality and nature (and even coherence) of some of the things people profess to mean by "consciousness."
~400 BC: Democritus proposes that all human-scale phenomena, including psychological phenomena, are the result of small physical parts bouncing off each other. From Encyclopedia Britannica: "Democritus thought that the soul consists of smooth, round atoms and that perceptions consist of motions caused in the soul atoms by the atoms in the perceived thing."
1641: René Descartes, Meditations on First Philosophy. Descartes argues that mind and matter must be irreducibly distinct (mind-body dualism), because (e.g.) material things are spatially extended, while thoughts are not. Descartes speculates that minds interact with the physical world via a specific part of the brain, the pineal gland.
Descartes also popularizes the idea that everyone knows their own conscious experiences with certainty: at any given moment, we are infallible about the fact that we are having an experience (the "cogito"), and we are also infallible about the contents of that experience.
1651: Thomas Hobbes, Leviathan. Hobbes insistently asserts that everything (including the mind) is material, and can be thought of as a mechanism or machine.
1714: Gottfried Leibniz. The Monadology. Leibniz argues that mind can't be reduced to matter:
"One is obliged to admit that perception and what depends upon it is inexplicable on mechanical principles, that is, by figures and motions. In imagining that there is a machine whose construction would enable it to think, to sense, and to have perception, one could conceive it enlarged while retaining the same proportions, so that one could enter into it, just like into a windmill. Supposing this, one should, when visiting within it, find only parts pushing one another, and never anything by which to explain a perception. Thus it is in the simple substance, and not in the composite or in the machine, that one must look for perception."
1866: Charles Sanders Peirce, Lowell Lectures. Peirce introduces the term "qualia" to refer to what it's like to have a specific experience — e.g., the particular experience of redness. Qualia is the plural of quale, Latin for "what kind of thing?" and source of the English word quality.
"The consciousness of brutes would appear to be related to the mechanism of their body simply as a collateral product of its working, and to be as completely without any power of modifying that working as the steam-whistle which accompanies the work of a locomotive engine is without influence upon its machinery. Their volition, if they have any, is an emotion indicative of physical changes, not a cause of such changes." And: "to the best of my judgment, the argumentation which applies to brutes holds equally good of men".
1888: Santiago Ramón y Cajal, "Estructura de los centros nerviosos de las aves." Using Camillo Golgi's staining method, Ramón y Cajal discovers that brains are made of neurons.
1903: G.E. Moore, "The Refutation of Idealism." The early 20th century saw sharp moves away from spiritualism and supernaturalism in intellectual circles, beginning with the "Cambridge revolt against idealism." Mysticism and metaphysical proclamations about the mind became increasingly unfashionable, as intellectuals grew more skeptical and more inclined to demand testable operationalizations of claims. Extreme manifestations of this attitude included logical positivism in the 1930s-1950s and behaviorism in the 1920s-1960s.
1968: David Armstrong, A Materialist Theory of the Mind. An early attempt to sketch a theory of consciousness (specifically, a higher-order theory). For an overview of popular theories or sketches-of-theories in the following decades, see SEP's review article "The Neuroscience of Consciousness."
1974: Thomas Nagel, "What Is It Like To Be A Bat?" Nagel writes that "fundamentally an organism has conscious mental states if and only if there is something that it is like to be that organism—something it is like for the organism." And:
"If physicalism is to be defended, the phenomenological features [i.e., what it's like to have certain experiences] must themselves be given a physical account. But when we examine their subjective character it seems that such a result is impossible. The reason is that every subjective phenomenon is essentially connected with a single point of view, and it seems inevitable that an objective, physical theory will abandon that point of view."
Subsequent authors have tended to use terms like "what it's like," "phenomenal consciousness" (derived from phenomena in the sense of "appearances"), and qualia to gesture at this apparent puzzle. These are closely related terms, used in slightly different ways by different authors.
1974: Robert Kirk, "Zombies v. Materialists." This paper introduces the philosophical zombie, or p-zombie: a hypothetical being that is physically identical to a conscious person, but lacks consciousness. If the idea of p-zombies has no hidden logical inconsistencies, it is argued, then consciousness is not logically entailed by organisms' physical properties, which would make physicalism false.
1982: Frank Jackson, "Epiphenomenal Qualia." Jackson argues that we can imagine a scientist, Mary, who knows all the physical facts about color but has never seen the color red for herself. If she then sees red, it seems as though she learns a knew fact—she learns what it's like to experience redness. Jackson takes this to mean that there are further facts beyond the physical facts, and that physicalism is therefore false. (For subsequent discussion, see SEP's "Qualia: The Knowledge Argument.")
Chalmers speaks of the "hard problem of consciousness," the problem of explaining why we are phenomenally conscious (i.e., why we aren't p-zombies). "Many books and articles on consciousness have appeared in the last few years, and one might think that we are making progress. But on a closer look, most of this work leaves the hardest problems about consciousness untouched. Often, this work addresses what might be called the 'easy' problems of consciousness: How does the brain process environmental stimulation? How does it integrate information? How do we produce reports on internal states? These are important questions, but to answer them is not to solve the hard problem: why is all this processing accompanied by an experienced inner life?"
While Chalmers discussed consciousness earlier (e.g., in 1993, 1994, and 1996), The Conscious Mind is the work that brought dualistic and quasi-dualistic views back into the intellectual almost-mainstream for the first time in a century. In spite of its crazy-sounding conclusions, the book is unusually clear, rigorous, and thorough, anticipating almost all of the obvious objections; and Chalmers attempts to make the irreducibility of consciousness more palatable to scientists by endorsing what he calls "naturalistic dualism": the view that consciousness is lawful, predictable, and not specific to humans. Chalmers argues that our consciousness depends on stable (but contingent) "psychophysical laws" that would also (for example) make a whole-brain emulation conscious.
2003. Max Tegmark, "Parallel Universes." Although not explicitly concerned with consciousness, Tegmark's picture raises problems for anthropics and our understanding of what makes an observer "real."
2008. Eliezer Yudkowsky, "Zombies! Zombies?" This and other posts from Physicalism 201 argue that we can be confident physicalism is true, even without knowing how to solve (or dissolve) the "hard problem of consciousness".
In particular, Yudkowsky argues that accepting the possibility of p-zombies is tantamount to accepting epiphenomenalism, and that epiphenomenalism is crazy. If our claims about consciousness are true even though consciousness has no causal effect on what we claim (because a p-zombie would move its lips and pen exactly as we do), then our claims would have to be true by coincidence, which is absurd given the Bayesian understanding of evidence and knowledge.
2008. Eliezer Yudkowsky, "Collapse Postulates." This and other posts from the Quantum Physics Sequence argue that physicists' belief that observers or consciousness play a privileged role in quantum phenomena is based on a series of confusions and misunderstandings.
2013. David Chalmers, "Panpsychism and Panprotopsychism." Chalmers argues that everything in the universe (down the the subatomic level) is "conscious" or "proto-conscious."
2014. Benya Fallenstein. "L-zombies! L-zombies?" Fallenstein asks how we can distinguish between instantiated observers and uninstantiated ("merely logical") observers.
2016: Keith Frankish. "Illusionism as a Theory of Consciousness." Frankish argues that "experiences do not really have qualitative, 'what-it’s-like' properties." Instead, subjective experience seems "unphysical" or "irreducible" because of a sort of introspective illusion.