Reposting my original question but this time, with a timefame!
Great question!
The intellectual pedigree of leftist thought can actually be traced all the way back to the philosophical traditions of the Greeks and early Christian theology. Plato and the Platonists, for example, examined the temporal condition of human existence, "degraded reality," and so on - for a further examination of these ideas, see the early chapters of Lesek Kowlakowski's "Main Currents of Marxism." Kolakowski is much more knowledgeable on these early thinkers and their connection to Marxism than I am, so I'll move on to the main point of your question - the 18th and 19th centuries.
Any intellectual examination of Marxism will have to account for Hegel and his place in shaping Young Marx and his materialist philosophy - but since I wouldn't necessarily call Hegel a "leftist" any more than I would Plato, we'll move ahead a little further and get into some others who more traditionally fit the mold.
Moses Hess (1812-1875) was a philosopher in the tradition of the so-called "Young Hegelians." Many of his ideas contributed to Marx's imagining of scientific socialism. In 1837, 11 years prior to the publication of the Communist Manifesto, Hess wrote of "a new era of Man's covenant with God," wherein a new society based on love and communal exchange would emerge. He expanded on this in his 1841 The European Triarchy, where he foretold of a coming social revolution based upon the growing disparity between wealth and misery. Hess's ideas on these subjects, though, are fairly elementary and undeveloped; for this reason, he is often associated with a strand of Utopian socialism that Marx was unflinchingly critical of - notably in his The Germany Ideology.
Ludwig Feuerbach (1804-1872) is probably more well-known than Hess, in no small part thanks to Marx's critiques of his thesis. It was from Feuerbach that Marx's materialist analysis stemmed (well, in a rudimentary form). He was particularly interested in the subject of religious alienation and objectification. Some of his writings on this issue later were reflected in Marx's understanding of labour alienation.
At this point, we start moving into a period where Marx is writing some of his early stuff - but instead of getting into that, we'll look at some of his contemporaries. But keep in mind, if we want a full understanding of the philosophical underpinnings of "socialism" in the mid-19th century, Marx must be roundly understood.
Max Stirner (1806-1856) was another of Marx's contemporaries who drew his considerable ire. To Stirner, the notion that man would serve society rather than individual Ego was a "triumph of generality over true existence." We would later see Nietzche draw upon Stirner's writings, although I'm not sure if he is explicitly cited. Despite this, Stirner did offer some cogent critiques of his society - largely, again, based upon inequality and misery. Marx, however, felt that his rebellion was adolescent and without substance; famously, he felt that Stirner thought the state could be destroyed intellectually, not materially.
If I'm not mistaken, Proudhon, LaSalle, and Bakunin have already been explored by other posters, so I'll skip over these philosophical giants. They really are very important for examining 19th century socialism, so I'd recommend reading into them further.
As far as I can tell, the first use of the term "socialism" came in 1832 in an article from Le Globe. It was used by a man named Pierre Leroux, a likely follower of Henri de Saint-Simon (1760-1825), another early pro-socialist who discussed the emergence of the industrial working class. He ultimately believed, as evidenced in his L'Industrie, 1817, that the methods of industrial management should be applied to all social questions in the process of advancing towards an "organic" future social community. He believed that production and exchange based upon private ownership was corruption; and that all political change has been the result of transitions in productive forces and capabilities. Here we see the seeds of historical materialism.
Owen and Fourier are, if I am correct, discussed above as well in another comment. There are plenty more, though - more than can be included in a single comment even! Hope you find this all interesting.
The answers so far, in my humble opinion, do a decent job of touching on some of the first thinkers/movements (utopian socialists and others) to formulate and develop "far-left" trains of thought on a large enough scale.
I'll add, maybe patch some holes regarding what is more clear cut, but I'd also like to develop a bit in the proto-socialist/(BTW, kudos for mentioning the Levelers)primitive communism side of things.
So, first I think you can't mention Proudhon without mentioning his most recognized predecessor, the first to call himself an anarchist, and often deemed first philosophical anarchist:
I also would make it a point to mention:
Who BTW was a harsh critique of Proudhon, and the first to use the term libertarian-socialism.
Though to make a point on Godwin and Proudhon, the former often regarded as the Father of anarchism, the latter as the Godfather, some would argue that it demonstrates reductiveness in a paternilistic and eurocentric sense, I would side with them.
No one should have to argue, that leftist thought (from an articulated philosophical perspective) was influenced by the enlightenment, that anarchism branched off socialism, but what I find problematic, which is defended by some in academia, is that anarchistic/socialistic ideas or praxis have no historical basis as far as being developed or enacted before the big name philosophers came in. Their thought's didn't materialize out of thin air, there was a precedent, albeit maybe not articulated as clearly, but there are precursors, and there are some who make it a point to demonstrate that as long as there has been oppressive institutions and exploitation there has been opposition (i.e., far left aspirations).
Disclaimer: by no means am I an historian, I just know a bit about radical history, and this is not meant to be exhaustive by any stretch of the imagination.
To more modern works, like that of anthropologist James Scott on anarchistic currents in South-east Asia
...There's actually a lot of substance in regards to resources pointing at instances of socialistic or anarchistic currents throughout the world, and through time.
I'll just note a couple things I have encountered through some readings, I encourage you to read yourself, I believe there's some work to be done in that area, that of trying to put a finger intersecting motivations, even if not self-defined, pertaining to proto-socialism.
The Great french revolution was definitely a hot bed of (what one could call) the development of radical leftist ideas, and those movements/elements are definitely worthy of their mention due to their contribution to proto-socialism/anarchism: the Herbertists, the Enragés and the Sans-culotte, to name a few...
http://www.britannica.com/EBchecked/topic/259019/Hebertist
http://www.britannica.com/EBchecked/topic/522646/sansculotte
http://www.britannica.com/EBchecked/topic/188651/Enrage
...Most definitely worth looking into...
I would also suggest reading Kropotkin's extensive and well sourced The great French revolution, which covers the oft forgotten, more radical/anarchistic elements of the revolution, which tend to get caught in the shadows of the often recounted (slightly less radical within their own rights, each, respectively on the spectrum of violence and political terror), perhaps more politically significant: Mountain, Girondins, and Jacobins.
Going down the path of older happenings, I would also suggest looking into Kropotkin's other works, both Mutual aid: a factor of evolution & Ethics: origin and development.
Mutual aid posits and demonstrates the proto-socialist reality of communal ownership and some historical precedents: it includes his intrinsic knowledge of the Swiss Jura Federation (Kropotkin was quite familiar with the reality due to his first hand accounts, spending a fair amount of time living with the watchmaker's associations, absorbing the culture), some accounts of indigenous cultures and how they went about dealing with land, for example, based on his experiences in Siberia, and he also dwells and expands quite a bit on communal notions within worker's guilds during the middle ages...
Now, Ethics can be seen as a follow up, most definitely can be read as such, it expands on the former, all the while tracing a sort of lineage of the development of philosophical thought, perhaps it pertains more to what you're looking for, if you are more geared towards thinkers.
It's less of a historical resource, definitely more so a work of philosophical dissecting, but it follows a developmental timeline, and it does a really good job of addressing Fourierism, Saint-Simonism, and Owenism in the big scheme of things.