One of the unique things about the Dreamcast was the VMUs. Memory units that could be plugged into the controllers to act as a second screen, or by themselves as a mini handheld gaming device (that is, if their batteries lasted).
Over time I’ve accumulated quite a few of the different colours. The transparent green, blue and black VMUs are American, whereas the transparent pink one is Japanese, hence the different colour buttons. I also love the carbon fibre one as it matches my Anthracite NeoGeo Pocket Color!