A simple 1-bit half adder that combines two 1-bit inputs to return a 2-bit output. It can't do very much, but it can save outputs to memory, read from that memory, clear it, and write again. I always use redstone torches for my interfaces but feel free to add buttons/levers.

Not the coolest redstone device, and also not my latest creation (this one's about 4 months old), but in my 4 years of redstoning, it is/was the first device I've ever created to have storage memory that is more than just RAM used for bigger ALUs.

Some misc info: This device was created in Minecraft 1.5.2, so if redstone mechanics have since changed, it may not work properly. I'm not educated on the changes made with later updates, so I wouldn't know myself.