DM42 is a modern clone of the HP-42s RPN scientific calculator. It is rather expensive and I don't have much of a practical need for it, but it's fun. (See also: Free42, the FOSS HP-42s simulator/implementation the DM42 uses. Run it on your computer or smartphone.)
For programs I've written for fun, see HP-42s on gopher.
DM42 supports screensaver images, displayed when the calculator is off. Images are 400x240 1-bit bmp. Below are the images I've added to my rotation. See also the stock images.
source: Principia Discordia
source: Principia Discordia
source: Copland OS wallpaper
source: PLT Scheme logo
source: NERV logo
source: Seele logo