Faber-Castell Clic & Go Water Cup is one of my best art product purchase recently.
This is actually a rubber collapsible cup! The size can actually fit a normal sized plastic cup inside. When collapsed, it's compact to bring around. The current size is fine, but I do wished that they could have made one that half the size to make it even more compact.
One thing I like about the cup is the wavy rim that allows you to put brushes horizontally. This is fantastic because now I can dry my brushes like this in the future.
This is smart design.