The Maelstrom Mod introduces four new races to the Sins of a Solar Empire Rebellion Universe. They are independent of the original races and have unique strengths and weaknesses. Maelstrom is available for Rebellion, Diplomacy, Entrenchment and Original Sins.
Fun, and adds alot of variety. The new planets have unique effects that can shift the balance of power quickly. The races are also unique, my favorite being the replicators and their ability to, well, replicate! Unfortunately, some balance issues seem to be present, but it's likely to be expected with 6 races. And some more polish is likely in the works. Given time, this could be the mod that makes you even want Sins of A Solar Empire ;D