alt Hacker NewsCuecat? Though I'm not sure if that ever really got big enough to warrant a spot, but it was a thing for a hot minute.
HomeRF - The wifi contender that was supposed to unify wireless networks across multiple device types.
Tivo - technically still around, but pretty much a zombie version of their former selves. (Maybe make an "I'm not dead yet!" section).
Gateway 2000 PCs -Their support line had a DJ, and they were everywhere until all of a sudden they were nowhere.
Optical Drives
Zip Drives - 100MB on a floppy! OMFG!!!11
Slashdot - Digg and Reddit's funky uncle.
Replies
Slashdot is still online and updating. Some of us still use optical drives from time to time (especially those of us with an existing stock of M-Disc media for long-term archiving).
Gateway was purchased by Acer, so it’s not like they just disappeared. Not any more than DEC, SBC, or Studebaker anyway. They were just absorbed.
Interesting Cuecat story: LibraryThing bought a massive stock of Cuecats and, 20 years later, they still sell them, repurposed to scan ISBNs from books for cataloging purposes: https://wiki.librarything.com/index.php/CueCat_Guide