Forbes has posted a gallery of ten of Apple's more ignominious flops. There's a good chunk of the usual suspects here: the Lisa, named after Steve Jobs daughter, buried en masse in a Utah landfill, the Pippin, the Newton. But there's also some forgotten gems, like Apple's vaporware "Taligent" OS.
I also found this observation on why the G4 PowerMac Cube failed to be interesting:
The PC's unique shape, a cube with a top-loading toaster-style CD drive, seemed poised to create a PC design revolution. Instead, Apple announced it was putting the machine "on ice," in a press release a year after the Cube's launch.
Apple's mistake in that case, says Kay, was depending more on Jobs' personal taste than market research. In a study Kay worked on as an analyst at IDC a year before the Cube's launch, researchers gave users blocks of foam in various shapes and surveyed them on which blocks they preferred and why. Kay found that users opted for "dramatic" shapes--those that had at least one dimension very different from the others.
The Macbook Air, for instance, with one extremely thin dimension, would have scored highly. But by the same measure, the G4 Cube "was exactly the wrong product," Kay says.
Apple Product Flops [Forbes]