The Romulans
The Romulans are much more stealthy than the other races, and one of their special abilities reflects that. If you are able to transport the spy unto an enemy ship after researching him, you see what all that ship sees in its sensor range. If you are able to transport him into an enemy installation, (assuming it isnt destroyed) you then have the same view that the player has over all of his forces. This, coupled with the ability of almost every Romulan ship to cloak, can lead to some very interesting ambushes. Beyond this, the Romulan forces are very similar in nature to those of the Federation.
The Borg
While the Klingons are good at taking over enemy ships via their shock troops, it is the Borg who truly excel at this action. One of the special abilities of the flagship of the collective, also known as the infamous cube, is to actually beam Borg onto the enemy ship, and transfer crew members off of that ship and into its own. This allows the cube to take over enemy ships with ease. Consequently, any ship that begins as an enemy to the Borg may soon become an ally. Another feature that is unique to the Borg is their ability to instantaneously transport ships to any location occupied by another Borg vessel via a transwarp gate. Thus, once you build this gate, all you have to do is to get one small scout vessel past the defenses, and your entire fleet can hone in on that ships signal and transport behind enemy lines.