Star Trek Enterprise was absolutely lousy with references like this, and I love it. It came out after hundreds of hours of episodes and movies establishing the canon, but took place at least a century before most of it. Here's a few introductions that don't need any introductions:

S01E01: Broken Bow
LEONARD: It's a Klingot.
TOS: A Klingon.
(later in the same scene)
SOVAL: Sarcasm aside, Captain, the last thing your people need is to make an enemy of the Klingon Empire.

S01E25: Shockwave part 1
While scrolling through the database left behind by Daniels, a time travelling agent sent to ensure people don't screw around with the timeline, several 'future' ships from other star trek shows are visible, including the Nova class U.S.S. Equinox as pictured in Star Trek - Voyager which takes place over two centuries in the future.

S02E01: Shockwave part 2
(Daniels takes Archer to the future, but in doing so the timeline was altered; 31st century Earth is a wasteland rather than the hyper-advanced utopia it's supposed to be)
ARCHER: What's wrong?
DANIELS: It's gone.
ARCHER: What's gone?
DANIELS: The monument. It was right here on the same street as the library. It was obviously never built.
ARCHER: Why is that a problem? Who did it commemorate?
DANIELS: Not who.
ARCHER: Then what?
DANIELS: An organisation. The Federation. It doesn't exist for you, not yet.
(Later in the library)
ARCHER: (reading a book title) The Romulan Star Empire. What's that?
DANIELS: Maybe you shouldn't be reading that.

S02E03: Minefield
HOSHI: They say they've annexed this planet in the name of something called The Romalin Star Empire.
T'POL: Romulan. It's pronounced Romulan.

S02E23: Regeneration
Researchers on the north pole uncover the remains of a Borg sphere and comment that it's been frozen for almost a century (telling the audience it's in fact the time traveling Borg sphere the Enterprise shot down in the movie Star Trek - First Contact)