Most people know it for the chapters set in Liliput (where Gulliver is a giant compared to the natives), and maybe also Brobdingnag (where the natives are giants, and it's Gulliver who's considered tiny). It's a scathing social and political farce, where Gulliver's visits to other societies show different systems of government and different social orders. While in Liliput, Gulliver is considered a dangerous giant, and treats the tiny locals with callous indifference. In Brobdingnag it's Gulliver who's overlooked and considered unimportant, so the extent that he's caged and treated as a pet.
But there's also a chapter (at the end) where he visits the Houyhnhnms, a race of talking horses that Gulliver considers entirely superior to humans, who regard the local ape-decended species (the "Yahoos") as loud, primitive, warlike and violent. After living with the Houyhnhnms, Gulliver comes to see all humans as Yahoos.
And for the science fiction fans, there's a chapter about a giant flying saucer.
Really, there is. The third section of the story has Gulliver being rescued by a scientifically advanced society, based on the flying island of Laputa. The city is built on a four-and-a-half-mile-wide concave circular plate topped by buildings, along with four lakes for collecting rainwater, surrounding an astronomical observatory built into a central shaft, that also includes the levitating mechanism. It is, quite literally, a "castle in the air" inhabited by scientists.
Laputa rules over a kingdom (Balnibari), whose borders are defined by the limits of a naturally-occurring geological magnetic anomaly, and the flying city is held aloft by a giant tiltable magnet, held in place by unbreakable "adamant" cage that is of a single piece with the city's baseplate. The city rises and falls and gets sideways propulsion by adjusting the alignment of the magnet. The flying city is a local scientific superpower, and the king's response to rogue cities below is to steer the saucer above the rebel stronghold and set it down, crushing them.
Unfortunately, I'm not aware of any illustrated editions of "Gulliver's Travels" where the illustrators tackled Laputa. Perhaps the idea was just too freaky for them. If they had, they'd have probably ended up drawing something that looked like the mothership in Spielberg's 1977 "Close Encounters" film.
The other notable thing about Swift's flying city of Laputa is that although it is ordered along entirely scientific principles, its (highly quotable) math-and-music obsessed inhabitants at the Academy of Lagado are buffoons, working on crazy and expensive projects such as the extraction of sunlight from cucumbers, constantly begging for more money for their projects as the society below them decays – it's a fairly small step to suspect that Swift was taking the mickey out of the esteemed Royal Society (then headed by Isaac Newton), and it's even been claimed that Swift emphasised this by basing all of the Lagado projects on specific Royal Society papers.
This raises an intriguing question: did Swift actually meet Newton? It seems that when Swift had been in London in 1710, he'd been visiting a woman called Catherine Barton. Barton was Newton's half-niece, and one of the few people that Newton was close to. Barton wasn't just some peripheral nominal relative of Newton, she'd actually moved to London and moved in with Newton in about 1696 (about the time he got his job at the Mint), and kept house for him.
If Catherine Barton was living with Isaac Newton and being visited by Jonathan Swift, then Isaac Newton would have cast a rather large shadow over Swift's consciousness, even if he hadn't been /the/ Isaac Newton.
And if that wasn't enough, there was also the subject of Money. Immediately before "Gulliver's Travels", Swift's celebrity was based on his having anonymously written and published the Drapier's Letters in ~1724-25, a series of pamphlets railing against the coining of copper currency for Ireland, which led to a widespread boycott of the new coins in Ireland and their withdrawal. One of Swift's (many) objections was an allegation that the coins were of poor quality - Newton, as Master of the Mint since the mid-1690's, had to get involved and do an assay, and reported that the allegation wasn't true. Newton was known for his tetchiness, but Swift in particular had a reputation for being gratuitously and grossly offensive. I've got an old C19th copy of "Gulliver's Travels" that describes Swift as having "more than any other man who ever wrote in English, a liking for saying nasty things", and blames this for Swift's repeated ruination of his own career prospects. Apparently Swift wanted to be a bishop, but even as a returning hero of the Irish people, when the people in charge actually met him, it became clear that this wasn't going to happen. That edition of "Gulliver's" mentions "the deadly agitations of his private life" as being something that the C19th reader might want to enquire about in later life – but whatever this unmentionable personal train wreck was, it doesn't seem to have made it as far as his Wikipedia page.
So perhaps the two wouldn't have wanted to meet each other, especially since they both cared about the same woman. Having the the brittle, acidic, reserved Newton in the same room as the extrovert, scandalous, offensive Swift might not have been a good idea, and the fact that they both had strong ideas about currency would probably just have made things worse.
The young, exiled Francois Marie Arouet ("Voltaire") was also in London around this time, and seems to have been rather keen on Catherine, too. Voltaire later went on to write "Micromegas" (1752), a short satire that appears to have been partly inspired by "Gulliver", in which a pair of giant aliens from Sirius and Saturn arrive on Earth and meet up with and ridicule a bunch of tiny Earth philosophers (with the exception of one guy who is a follower of John Locke). That's John Locke, the guy whose writings seem to have influenced the American Declaration of Independence, not John Locke, the character from "Lost" (a TV series about a strange island with a natural magnetic anomaly).
Another link between Voltaire's story and Swift's is that both throw in a little casual detail (known to the fictional Laputans and alien scientists) that the Mars was "known" to have two moons, and it seems natural to assume that Voltaire probably borrowed this detail from Swift. In fact, Mars has got two moons – Phobos and Deimos – but they didn't get discovered for real until 1877. That earned both writers an astronomical "credit": the only two named features on the smaller of the two satellites are a pair of adjacent craters, named "Voltaire" and "Swift".
After he'd given up on the brilliant Catherine and snuck back to France, Voltaire shacked up, long-term, with another brilliant woman obsessed with Newton, Émilie du Châtelet, who as well as being a serious respected researcher in her own right, translated, produced and reworked (with her comments) the French edition of Newton's Principia.
To the C18th coffee-house intelligentsia, a mix of physicists, philosophers and political theorists, this was a time of revolution and restructuring (not to mention a certain amount of fluidity over people's living arrangements). England had recently undergone a rapid turnover of rulers, flip-flopping from Monarchy to Republic, and back to Monarchy again, then Monarchy chosen by Parliament. Cromwell had kicked out Charles I, Charles II and James II had taken over from Cromwell, and the Glorious Revolution had then given Paliament the right to choose the monarch, which brought in William and Mary, and which they then exercised again in the Act of Settlement to shunt the succession to Anne, who'd then died, too. The Acts of Union in 1707 had then finally united England and Scotland as a single kingdom. In the politics of 1726 England there were various entrenched factions with specific ideas about how the country ought to be run, and by whom, but there was no guarantee that any one particular group would obtain ultimate control.
There was a sense that this was where we decided what the future was going to look like. Was it going to be run by royalists or republicans? Theologians or scientists? Committees or street campaigners? "Gulliver's Travels" tapped into an appetite for exploring possibilities, and showing how different systems failed. Voltaire's later story got a charge out of lampooning philosophers because at the time, philosophy was reckoned to matter. These guys were potentially the architects of the new society.
"Gulliver" can be seen as parody of how people brought up in different political and philosophical systems can believe that their own way of seeing the world and the correct order of things is right and proper, even when outsiders can see that it's ludicrous, and was, in a real sense, revolutionary. Together with a surrounding body of other philosophical and campaign literature, it helped to set up the context for debate that made the French Revolution and the American War of Independence seem possible to the people who risked their lives to make those things happen.
It's not just a kid's story about a shipwrecked guy being tied down with string by little tiny people.