Sisyphus was a super crafty dude who made the headlines of classical literature by outsmarting Death and for paying big time for his shenanigans. More than anything else, people probably associate Sisyphus with his eternal punishment: rolling a giant boulder up a hill for all eternity.
Plenty of classical authors include cameos by Sisyphus in their texts, especially when their characters cruise through the Underworld. For example, in Homer's Odyssey, Odysseus spots Sisyphus rolling his rock, and in Ovid's Metamorphoses Sisyphus takes a break from his labor to listen to the song of Orpheus. You can find bits and pieces of Sisyphus's story in the works of big deal writers like Apollodorus, Hyginius, and Diodorus Siculus.
Over the years, plenty of philosophers have found inspiration in Sisyphus's endless task. Most famously, Albert Camus's "The Myth of Sisyphus" uses Sisyphus's labor as an example of the absurdity of human existence. In Camus's mind, there's no greater purpose to anything that any of us can ever do, therefore making it all just as absurd as pushing a rock up a hill forever. This might sound depressing at first, but there's more to it than that. These days the term "Sisyphean task" is used to describe any activity that might seem endless and pointless.
Don't go thinking Sisyphus only pops up in philosopher's books, though. More recently, he's inspired songs by such classic rock bands as Chicago and Pink Floyd, been a reoccurring villain on Xena and Hercules, and made a couple cameos in the Percy Jackson books. Hey, not too bad for a dude who's stuck in Tartarus for all eternity.