Interesting article, and I must say that I didn't know a couple of the models the author deem as a "flop". Here's the list, but I recommend visiting the article, because of the pictures and screenshots:

1.Apple Newton
2.Magic Cap
3.GEOS based PDA
4.Palm PC
5.Motorola Marco
6.VTECH Helio
7.Royal LineaLX
8.GRiD PAD
9.HP 200LX
10.Xircom Rex

I'm not sure if some of these are flops, or just their time is gone. Good descriptions and interesting comments.