Picard discusses the trial from the pilot episode.
Picard mentions the time Q offered to make Riker part of the Continuum.
Q mentions the settlements that were "scooped" out of the planets.
The Enterprise's first encounter with the Borg at system J25 is mentioned a couple times.
Guinan's back story with the Borg becomes an important part of this confrontation.
Janeway mentions that Q was responsible for Starfleet's encounter with the Borg.
Pinder compares Infeld's office building to the Borg
Jesse St. James tells the Vocal Adrenaline kids that they're the Borg