My Commodore 64 with the Summatech 256Kb REU installed. This REU is not compatible with the "real" REU, however it is a nice little hardware still. It has an own Basic, can realize a RAM Disk, however otherwise it is quite simple. It only provides access to it's RAM in a single register with an auto-incrementing address. Well, you may not mess with the VIC with this thing, however for any sequential access oriented algorithm this can be very useful and efficient, while the hardware is simple.