I don't think RDRAM and ISDN qualify for this list. There are people that still try to obtain RDRAM motherboards and memories and do some builds. On the other hand, ISDN modems are not forgotten but mostly useless today (I mean, people don't buy them because they can't be used anymore).
Two more contenders...
Computer/monitor/keyboard/printer dust covers: Back in the 80s, PC and XT clones were cheaper than their IBM counterparts but still an expensive item in most homes. So they should be cared to avoid early failures... what about covering them so no dust get inside? Still you can buy some new or NOS computer covers in some shops... but I guess that nobody make an XT build and buys some kind of cover to get that "correct era" look. On a second thought... do you use this covers when you have computers in storage?
Screen filters: If all those stories about radiation coming from CRT screens were true, most of us would be blind or be a green glowing mutant. But there was a time when this filters were on almost every CRT display was equipped with one of those filters (I mean the "transparent" kind, not the privacy one). Attributes: radiation -??, reflections +4. Yes, the most visible effect of this filters was that they were slightly less reflective than a mirror. You could see everything at your back... and most sources of light (windows, lamps) were annoying. What are your approach with screen filters? Do you buy them for your old CRT monitors? If you buy a CRT and get a filter... do you keep it or trash it? In any case, it's funny how they are returning in the form of "anti-blue light" screen protectors...
Bonus track:
Anti radiation plants: It seems that not only our monitors produce harmful radiation, but computers do as well. So some years ago, many gardening and flower shops were selling plants that supposedly fought these radiations. There are many kinds, but most of them were of the cactus kind. Anyone bought one of these? Did you need different plants for AMD or Intel computers?
The unforgettable one:
A cat over your monitor: This items can not be bought anywhere but can be easy to obtain, and there are many models to choose from. A CRT monitor is a warm (mostly) horizontal place to rest, and most cats feel comfortable with them. When I was a kid, I had a ZX Spectrum and my cat used to be with me on my gaming sessions. It was so lazy that my computer shut down routine included removing it from the TV. Later, my girlfriend also had one, but when she bought her first LCD screen discovered that the hardware was incompatible (it was too thin for a cat).
EDIT: I've added more photos to the first post... how can I resize those photos so they don't break the thread?
I have traveled across the universe and through the years to find Her.
Sometimes going all the way is just a start...
I'm selling some stuff!