There have been various references over time about technology that's been captured from a rival power. FASA mentioned a number of enemy vessels that had been captured by each of the major powers, along with the Orions (whose fleet was comprised mainly of Klingon designs, some of which were acquired illicitly). I would guess that two of the more valuable technologies would be cloaks and Borg equipment, assuming the later could be integrated into a useful system as Voyager managed.
What do you think about technology that's been captured and reverse engineered?
What do you think about technology that's been captured and reverse engineered?