Too many to choose from. The elcassette should certainly be on the list, though I don't have any of those. I do have a couple of DCC tape decks (anybody remember those?) and three portable CD-I players (very much like modern handheld DVD players, but much clunkier and more expensive). Don't have any CD-I disks, however.
I used to have some Videotrax backup systems. It was really quite clever: it modulated the data onto a video signal and let you use a VCR as an amazingly cheap (especially for the time) streaming backup system. Never caught on that much, but the manufacturer even tried the experiment of renting time on cable TV channels to distribute software and other material with this system.