i am wondering what the most popular shield is for arduino, ethernet / wireless / motor / ...
(so what would be a best next toy to add to the arduino, to learn from)
If you are anything like me... It would probably be a 320X240 Graphics shield, Matter of fact at Electrodragon you can buy a 3.2" Itead 65K color shield for about $14.00 - $15.00 with touch and an SD card... Makes a natural for a Picture frame... just a small sketch to read pictures from the SD card and display them for about $30.00 in new parts one could have a 2.8 X 2 in picture viewer...
Doc
I would guess that the most popular are the prototyping shields...
What's the value of knowing the popularity? Motor shields are popular with people doing motor projects. Ethernet shields are popular with people doing network projects.
Decide what you want to do or learn next, then get the appropriate hardware.
I am guessing you already know about this (a bit dated) site http://shieldlist.org/.