3D slide viewers - Most had at least a vauge resemblence to binoculars. The most popular brand had seven image pairs on a disk, but another had rectangular cards with the image pairs down the sides. toy departments used to have a sizable rack of media for these viewers with at least a hundred different titles. There were even souvenir packages sold at many popular tourist attractions. There was even an original series "Omega Glory" set.
I still have one of those tucked away somewhere....
The Super 8. Very popular in the day, non-existent now
For another obselete technology - when was the last time you saw a home movie camera of any type?
Same with microfilm, microfiche, and (especially) microcard.
Also, microfilm and microfiche are still very much alive. Go to any university library or archives and you will see this. At the job I just recently left, we were still actively converting documents to microfilm daily, and people were coming in to view them daily, for reasons ranging from geneaological research to lawyers investigating legislative intent. You can argue that these mediums aren't mainstream outside of the research community, but were they ever?
Libraries still purchase items on microfilm/fiche and probably will for a little while. Digital technology is still a little too new and untested. We library people are very, very resistent to change in some respects, while embracing it in most others. Until one can show that computers and digital documents have sustained availability (library will not lose access to it) there will still be a market for microfilm.
Filmstrips and filmstrip projectors: These classroom staples have long since been replaced by televisions, VCRs, and DVDs.
Rotary dial telephones: Once a staple, now only seen in a museum. These were replaced by phones with number pads somewhere around '82.
Speaking of phones, when was the last time that you saw a phone that was hardwired to the wall with a non-removable cord?
Mimeograph machines: Ugh. I used to hate these things. You'd get a handout from the instructor printed in some godawful purple or blue ink that was hard to read because it was so badly smudged. Long since replaced by Xerox machines and later laser printers.
CB radios: These used to be in every store. When was the last time you saw one that wasn't in a semi? Even the truck drivers have gotten away from these.
Daisy wheel printers: Yes, there was a printer technology that dot matrix replaced...
Betamax: In all fairness, this was a niche technology even when it was popular. It lost to VHS and disappeared much like HD-DVD is now.
"Portable" VCR: I use quotes here because this monstrosity really wasn't what I'd call easily portable. Basically it was a VCR that you could remove the tape deck portion from and carry around with you. The deck portion could be powered by the wiring in your house or by a battery inside the unit itself. Unfortunately, the unit we had was like 50 pounds by itself. Also required a completely separate camera which would weigh another 40 pounds. Replaced long ago by the camcorder and with good riddance.
Dial tuners: Tere have always been television remotes. Unfotunately for us, back in the days before handheld remotes, the remotes were known as children. Channels only went up or down one step at a time and made a loud "ker-chunk" sound when they did.