Not necessarily in order.
- Data tape drive
$650 IIRC, $50 per tape that stores 1.6 GB (3.2 claimed with compression). I only ended up buying 4 tapes, and one of them was faulty but I never knew this, making for a terrible experience.
- CD rewrite drive
$16 per disk (at first) and often get unreadable data
afterwards. I gave up on this tech once portable hard drives came out and got cheap enough, and memory fobs.
- BlueRay player
Only used it once, maybe twice.
- Many of the WindowsCE devices I bought. Spent many thousands total over a few years, always hoping the latest one would finally be good enough (never was). One HP device with a keyboard cost me $1600 for example (again IIRC).
- Hand scanner
For digitizing paper documents. 256 grey scale colors, and while scanning, the software would often crash. Also needed a steady hand, though I had some success using a drafting table and its "arms". Todays table top scanners are wonderful in comparison.
I'll finish this list later.
Do you have any regrettable purchases for the cost and the little value you got out of the purchase?