“Technological device” or "technological knowledge?
Replicator was my immediate thought also, although if allowed, a 24th century datapad with instructions for engineering all of it’s tech would be even better. Or even a download of the wikipedia of any future date.
The standard autodocs don’t rejuvenate; Known Spacers use boosterspice (which is probably similar to the active ingredient in Tree of Life) for that. The Wu autodoc, however, will regenerate you from anything up to and including decapitation, with a brand-new body. Unfortunately only one exists.
If you really want to eliminate unemployment, I do recall some technology in a sci-fi novel that suppressed electricity. Every human being would immediately be employed for sheer brute force labor, and if they couldn’t manage they’d die and get eaten. Win win!
Surprised I’m the 1st to suggest: A Delorean equipped with a flex-capacitor.
You’ve impressed me enough to use mine and retroactively post this idea for #27, back on page one. 
Meh, not a DeLorean. Something with more style.
The only useful Star Trek thing I could think of was transparent aluminum. And the best part is, I’ll get my name in the history books for having invented it.
You want to invent reality television?