If you import a save from Mass Effect 2 with a live Prejek Paddlefish, take care of it carefully! Keep it alive through an entire Mass Effect 3 playthrough AND a Mass Effect 3 New Game+ playthrough you can get a special Intel Bonus from Liara's Quarters on the Normandy. The Prejek Paddlefish Bonus gives you a 10% bonus to either Weapon Damage or Power Damage.
Here are the bonuses you can get from importing a Mass Effect 2 save file.
Power on the Kingdom of Amalur: Reckoning demo to unlock the Reckoner Knight Armor and a Chakram Launcher weapon in Mass Effect 3. The armor "will maximize damage done in close-quarters combat while a beefed-up power cell feeds energy into weapon systems to increase projectile velocity," and that the launcher "uses a fabricator to manufacture lightweight, explosive ammunition discs."
You must import a Mass Effect 2 save in which you turned Legion over to Cerberus. In Priority: Cerberus Headquarters as you fight through the Reaper Heart/Brain room you'll come across a number of Cerberus Nemesis and a Geth named "Legion Assassin" that has the same appearance as Legion.
You must import a Mass Effect 2 save with Jack alive for this Easter Egg. If you don't rescue Jack in Grissom Academy: Emergency Evacuation before Priority: Citadel 2 she will appear as a Phantom in Priority: Cerberus Headquarters. The lifebar for a Phantom will simply be renamed "Jack" and she will say "I will destroy you!"
On the side-quest where you evacuate some Cerberus scientists, the AA-Turrets fire a path for the evac-shuttle through enemy fighters, then a voice announces, that the first evac-shuttle has sucessfully started. This is followed by a cheer of the remaining crew, like in the Hoth scene in Star Wars V.
You can actually recover your space hamster from Mass Effect 2. Head to Engineering on the Normandy and go to the lowest level via the stairs. You may hear some squeaking here. If you are fast, you can catch your space hamster for your quarters. Squeak!