Bit of a left-field recommendation, but how about The Suspicions of Mr. Whicher by Kate Summerscale?
It’s a factual account of a victorian murder, but it touches on the development of the murder mystery genre, early policing and victorian social history.
If you want something Olde School, try The Count of Monte Cristo (which bears little resemblance to any of the films due to its length and complexity). There are plot arcs so long that they go into Low Earth Orbit. Every time you think something’s going to be resolved, another new character (and character arc) is introduced. That everything ties up neatly in the end is a testament to Dumas’ skill, but it’s blatantly apparent that the man was getting paid by the installment.
Since the author who immediately came to mind on reading the title is mentioned in the OP (Neal Stephenson) and Discworld is recommended in the first reply I’ll offer something slightly different:
It’s not fiction but it’s certainly long and fascinatingly rambley.
There is David Foster Wallace and if you like fantasy, R.R. Martin (http://www.georgerrmartin.com/) has written a series of novles that read like the world’s longest prologue.
Mark Danielewski’s House of Leaves may be up your alley. It’s a long and rambling book with layers of “editors” making comments, big footnotes, and interesting digressions everywhere. The base story is about a family living in a house that starts showing extra-dimensional spaces, and the book itself essentially begins to do that as well, as more and more text/information/layers of meaning getting crammed in from every direction. By the end of the book you’re not sure if any of it is “true” from the perspective of people in the story, even though you’ve just been told this story from many different viewpoints.
Anything by Barbara Kingsolver, especially Poisonwood Bible or Prodigal Summer.
She describes her world with breathtaking beauty. Come to think of it, until I get the books I just ordered from this thread, I think I’m going to reread PS myself. It’s been a long time - I’d like to visit her again now that I have my own garden.
You could try The Gormenghast Trilogy. It’s kind of a crazy book.
I also second The Wind-up Bird Chronicle, House of Leaves and Cloud Atlas (though that one’s not that long)
And there’s always Tolstoy too - Anna Karenina and War and Peace
Tom Wolfe A Man in Full - not cluttered or crazy but long and a good read
Moby Dick? I know it gets a divided response but I liked it
Oh and the longest of all (7 volumes) is Proust’s In Search of Lost Time
I just want to third Tristram Shandy by Lawrence Sterne - you’ve basically described it already. Sterne anticipates post-modernism 200 years ahead of its time. The “story” is intentionally absurd, with endless digressions and digressions within digressions. He even messes around sometimes with the visual appearance of the text, especially when a character called Yorick dies.
Excellent thread, everyone! You’ve touched on some things that I’ve read and loved (Gormenghast, Barbara Kingsolver, Catch-22, House of Leaves), so I guess my OP was pretty accurate and I do, in fact, know what I like. This thread’s getting bookmarked.
Now carry on…
I came to recommend Drood. I didn’t like it myself, but it seems to match your request.
Middlemarch by George Eliot. It’s cluttered and long. Not cyber-punk, although if looked at a certain way…
Vanity Fair by Thackeray is also long and cluttered.
Can’t help you on the topical digressions, but Victorian authors are pretty much the definition of digression. 
I see someone mentioned David Foster Wallace already.
For SF I’d add Vernor Vinge.
Non SF, try Wilton Barnhad’s Gospel. Huge book about a search for a lost gospel.
Dear God, yes. If you like footnotes, then Wallace’s footnotes take up a measurable thickness of his books and almost need their own footnotes.
Like Dijon Warlock said, Foucault’s Pendulum, by Umberto Eco. Also, The Name of the Rose, and The Island of the Day Before. Eco has diverse interests, and loves to tell you about all of them.
Tolstoy’s War and Peace is a very entertaining soap opera, interrupted from time to time by a bunch of sermons on the Meaning of Life.
Try the musketeer novels of Dumas.
The Viscount de Bragellone is so long and convoluted, that most English-language publishers break it up into three installments:
3a. The Viscount de Bragellone.
3b. Louise de la Valiere.
3c. The Man in the Iron Mask.
I would recommend the Oxford paperback editions. They are heavily annotated, so when Dumas starts name-dropping, you can look up the celebrity he was talking about.
I’d recommend “A Winter’s Tale” by Helprin – it’s probably one of the classic urban fantasies and it’s massively convoluted.
Also, you might want to check out Infinite Summer, a mass reading of Wallace’s Infinite Jest. Here’s a quote from that web page:
Sound cluttered enough?
Oooooh…if I ordered the book right away, I could even start the mass reading on time.
Usually I discover these things a year and a day after they’ve finished.
Second Perdido Street Station or anything else by China Mieville. His writing is, well, rich in detail. Some of the details includes the most horrible things I have ever thought about, but there you go. I love his work.
Dickens is the master of that sort of thing. Bleak House is probably the best example.
Rabelais’ Gargantua & Pantagruel, Sterne’s The Life and Opinions of Tristram Shandy, Gentleman, Cervantes’ Don Quixote. They are all rambling, crazy, humorous and somewhat surreal - especially Gargantua & Pantagruel. Great book!