[Suicide Junkie]

I'd say most people play Unmodded, with TDM AIs.

Pirates & Nomads:
1) Great for turn-based Tactical combat lovers. As a Pirate or Nomad, you need to use every trick and exploit in the book to survive against the AIs, steal ships and tech, hide in storms, and raid unescorted transports.
Small pirate fleets enhance the "up close and personal" effect.

2) A unique form of tech balancing and expansion. Armor, shields, Phased weapons, missiles, racial crossover techs, added racial techs and traits, etc. Gobs of smaller changes.