The Ultimate Cart is an SD-card based multicart for the Atari 8-bit, which I've been developing for the last few months. It allows you to browse the SD card on the atari and launch cartridge images from ROM and CAR files from the menu. The PCB is designed to fit in a standard grey cartridge case (after a bit of modification with some clippers and a file).
It supports most ROMs up to 1 megabyte in size, including standard 8k and 16k cartridges, XEGS, AtariMax 1 & 8mbit ROMs, SIC! Cart, Bounty Bob, OSS, SDX & Williams. ROM dumps can be easily converted into CAR format with my html/javascript conversion utility, RomToCar.
New firmware also allows XEX files (Atari executables) to be launched direct from the SD card.