It used to sit under the iMac and provided a bunch of legacy ports.
The one I recall was called the iDock - course, searching for that name now shows up mostly iPod accessories. There were a few different versions, with a mix of floppy drive, serial, ADB, SCSI, PC serial and parallel printer ports. IIRC, they had driver stability problems. Yes, USB to the iMac, and they also functioned as a swivel/tilt base.
Griffin gPort and Stealth Serial Port
IIRC, these (and possibly the Keyspan) are the only solutions that work with Localtalk and MIDI.
