[Junior]

Believe it or not, Gulliver's Travels and Cyrano de Bergerac actually qualify, in some respects, as science fiction; both are definitely social commentaries...

[Doctor Stochastic]

As do Alice in Wonderland and Through the Looking Glass, althougn these are more akin to fantasy.

[PatrickHenry]

Among early SF writers (ignoring some really ancient stuff -- like the Odyssey) are Washington Irving (the guy who woke up 20 years later, a kind of time travel story); Mark Twain (Conn. Yankee, definitely time travel); and Charles Dickens (Christmas Carol, also time travel, but without the "machine" which HG Wells supplied later). I think Hans Christian Anderson did one about seven-league boots and some kind of consciousness swapping, but that may have been Dickens. Oddly, I can't think of any SF written by Poe. Anyone?