A VMU, or Virtual Memory Unit, for the Sega Dreamcast
Sonic Adventure 2 for Dreamcast is able to interface with the VMU, a small memory device with a screen designed for saving data on the Dreamcast. The VMU was able to connect to the Internet, and on a few occasions was able to download DLC for Sonic Adventure 2.
Sonic Adventure 2: Battle for Gamecube is able to interface with a Gameboy Advance containing a copy of Tiny Chao Garden, and can trade chaos with it.