I only have two:
A Confederacy of Dunces - Stopped halfway through. I suddenly lost interest in, well, basically everything. He was like fifteen paragraphs into talking about hot dogs when I suddenly decided I no longer gave a damn. The only other person I know who has read this didn't finish it either.
Mostly Harmless (I think? It was the 5th Hitchhiker's Guide book) - After reading the fourth one, which was pretty ehhhhnn, I didn't really have any steam left. I don't remember where I stopped. I don't even remember what was going on. I remember the part where
but other than that, everything after the second book sort of runs together. I read them too fast, I guess.Marvin died and they saw God's final message to creation (though I don't remember what it was)
Edit: Okay, I just skimmed the HGttG Wikipedia article, and I remember everything a lot more clearly now (so disregard the part about them blurring together). And it turns out the part I spoilered happened in the fourth book. It also turns out that I got farther in the fifth book than I remember (and, hell, I might even have finished it! I'm not sure).