I thought I'd post to announce my new hardware project - I'm building an SD-card based Multicart.
Although its far from being finished, it already shows the available ROMs/CARs on the SD card on an Atari menu, and will then reboot to the selected cartridge. At the moment it only supports 8k ROMS and 8-mbit Atarimax ROMs, with bankswitching etc, but I plan to support all cartridge types soon.
Adding a new ROM is simply a case of copying a new file to the SD card - no more flashing ATRs etc.
Not sure if there's any other available hardware that currently does this? I don't have an SIDE or MyIDE.
The hardware is a Altera Max 10 eval board, with my 3.3v cartridge breakout board attaching it to the atari. There's an external 1 megabyte SRAM which cartridges are copied to. Initially the atari boots to a small 8k boot ROM which is stored in dual port memory on the FPGA. The FPGA is also running a soft-cpu to copy files from SD card to SRAM.
I'm planning to make a PCB for this next, with the aim of fitting inside a standard cartridge case. Never done an FPGA board before, so that may take me some time. The boot ROM is also far from finished - I'm having to learn 6502 from scratch.
There are some other possibilities for this too - the same cartridge could be re-programmed e.g. as an atari co-processor/accelerator board. I'm planning to leave a JTAG/USB Blaster header on the board to make this easy.