Long thread, don't know if anyone's mentioned Thomas Pynchon yet. Gravity's Rainbow and V are popular titles of his.
Anything by Kerouac or at least anything belonging to the Duluoz Legend (ie: On the Road, Dharma Bums, Visions of Cody, Big Sur, etc) They're all pretty amazing. I'm a big fan of Big Sur, the place, I spend a lot of time there and the book has a special resonance for me. On the Road, I don't think anyone interested in literature of ANY kind should go without reading. It is essential reading for anyone interested in the concept of reinventing form, or disassembling it or re-approaching it in a different way--he invented something NEW, never done before with this novel, and only never done before, but it worked and it is beautiful. Very sentimental writer. A mythologized anthology of his own adventures, modern day, nihilistic lord of the rings. Though minus the nihilism depending on what book you read. Kerouac's philosophy and his interpretation of Eastern traditions fluctuated throughout his life and therefore in his books.
William Burroughs--Naked Lunch and Junkie. Kerouac and Burroughs also collaborated on a book together, the name already mentioned in this thread.
James Joyce. Finnegan's Wake will drive you CRAZY. I've been trying to decipher it for years. Also Ulysses is probably his best known title. It's about the day in the life of the protag, Stephan Dadelus, his day mirroring the events in Homer's Odyssey.
Jorge Luis Borges wrote a collection of short stories called Ficciones, which is BRILLIANT. He's obsessed with labyrinths, time, mirrors, and just all around paradoxical situations and heavy symbolism. He writes mostly metafiction, fiction about fiction. Stories about infinite libraries and impossible works and meeting himself near a river. Read this guy. Argentine author.
John Kennedy Toole--A Confederacy of Dunces. I haven't read this, but I've been urged to read it by a good friend, so I'm passing on the recommendation.
Someone already recommended House of Leaves and I am seconding that read. I read the second half of it while in a mental hospital, which made it doubly maddening to read. That book, a warning, can PUT you in a mental hospital. Perhaps the first half DID put me there.
Dark Tower series by Stephen King. I'm not a Stephen King fan nor a horror fiction reader, but this isn't typical Stephen King nor horror (though there's some horrific scenes in the books). It's a seven part series and it's basically about this guy, Roland, a gunslinger, who is chasing 'the man in black' across the desert. It's an alternate reality, wastelands, portals into other minds ala Being John Malkevich, drugs, western archetypes, fantasy, EVERYTHING. Stephen King started writing the first couple books in the 70's and completing the series the last couple I believe in the mid 2000's. I've only read up through the third one so far, but they're amazing.
If you like Hunter S. Thompson and Keasy, then you might appreciate Tom Wolfe's The Electric Kool-Aid Acid Test. I'm sure you've heard of it. His writing style is nothing like Hunter's, but he did use Hunter's recorded tapes of the Pranksters/Hells Angels fiasco to obtain accurate information.
Somebody already recommended Vonnegut, I second this. Slaughterhouse Five (temporal loops) and Sirens of Titan (spacial loops and future societies) being my favorites.
Camus and Sartre's fiction is good. Nausea (Sartre) is a trip and he wrote a play, No Exit, which is very interesting. Camus wrote The Stranger, which is an incredible read, a quick read also.
Kafka's later works are incredible.
Poe, obviously.
Faulkner. William Faulkner. Not really in the same domain as the others I've mentioned, or so it might seem. Light in August is an incredible experiment with non-linear narrative, which tells a gripping tale of the intertwining of three lives, well a good deal more than that, but the focus is on three main people, all told from each of their perspectives and, like I said, non-linearly, giving pieces of the story away at well engineered times that make it a unique read. I've never read The Sound and the Fury all the way through, but I didn't stop reading it for any lack of interest. I'd recommend that one as well.