Unity

The Unity addon provides each race with several options to backstab his enemy or support his economy. The examples listed in the addon's documentation are amorphous support, remote minefield laying and free torpedo building.

Next to the race-specific tactical options, Unity allows players to change the name of their race and the names of their planets via ingame messages. New friendly codes are introduced to better the effectiveness of the Merlin and new bdX codes make it much easier to beam down a specific amount of money to a planet you do not own yet.

The natives play an important role in the Unity universe, granting native gifts to colonist who treat them nicely. However, natives now contact eachother. Overtax the Amphibians on one planets, and the Amphibians on your other planets will get angry with you too. And somehow the Amorphous natives have been put to use too, providing free defenses and starbase repairs in return for eating your colonists.

With the Unity addon, the Federation gets the ability to shift a planet's axis, resulting in better mining conditions. The Lizard can lay minefields up to 150 lightyears away, the Crystals can now scoop up materials to build torpedoes from deep space by just flying around. The Robots can now detonate their minefields, damaging all ships inside the field even if they don't actually hit a mine. The Colonies have perfected their connection with Humanoid natives, and can now use those natives to influence behaviour of human natives on enemy planets.

All in all, Unity seems to add a lot of very interesting features to the game, providing some powerful options and enhanced gameplay.

Additional information: Unity Docs & Unity Tips