Listmania!
Massive Novels
By an Amazon.com customer
Clarissa: Or the History of a Young Lady (Penguin Classics)Clarissa: Or the History of a Young Lady (Penguin Classics) by Samuel Richardson
Buy new: $17.16 / Used from: $3.04
The biggest.  Not a novel....more of a monstrously large tome.
War and Peace (Penguin Classics)War and Peace (Penguin Classics) by Leo Tolstoy
Buy new: $10.88 / Used from: $0.99
As famous for being long as it is for being great.
Atlas ShruggedAtlas Shrugged by Ayn Rand
Buy new: $8.99 / Used from: $2.50
What a great load of crap this book is.  Still, it's looooooong.
A Suitable Boy: Novel, AA Suitable Boy: Novel, A by Vikram Seth
Buy used from: $0.74
I know very little about Vikram Seth, but what I've heard intrigues me.
Infinite Jest: A NovelInfinite Jest: A Novel by David Foster Wallace
Buy used from: $6.00
Huge, but more than worth it.  Hilarious stuff--a fantastic, illuminating read.
The Lord of the Rings (50th Anniversary Edition)The Lord of the Rings (50th Anniversary Edition) by J.R.R. Tolkien
Buy new: $53.55 / Used from: $42.50
Regardless of your opinion on its literary qualities, this is undeniably a one-of-a-kind achievement in literature.
The Stand (Modern Classics)The Stand (Modern Classics) by Stephen King
Buy used from: $2.18
One hell of a ripping good yarn.  King's greatest novel.
Gravity's Rainbow (Classic, 20th-Century, Penguin)Gravity's Rainbow (Classic, 20th-Century, Penguin) by Thomas Pynchon
Buy new: $12.92 / Used from: $2.99
The Big, Scary Novel of the second half of the twentieth century.  Don't let that frighten you--this is a dense and beautiful work.
The Recognitions (Penguin Classics)The Recognitions (Penguin Classics) by William Gaddis
Buy new: $16.50 / Used from: $10.95
One of the more fascinating books I've ever read--doesn't even seem to be 1000 pages.
DhalgrenDhalgren by Samuel R. Delany
Buy new: $12.89 / Used from: $2.50
The SF tag is a misnomer--there is a great deal more depth and interest in this novel then that label would imply
UlyssesUlysses by James Joyce
Buy new: $12.21 / Used from: $3.98
Best novel of the century?  I doubt it.  One of the best?  Maybe.
Remembrance of Things Past Volumes 1-3 Box SetRemembrance of Things Past Volumes 1-3 Box Set by Marcel Proust
Buy used from: $49.99
I'm scared to read this--although, like Finnegans Wake, it's one of those I'm going to have to read someday....