Sid Meier's Alien Crossfire is an outstanding (and unfortunately the only) expansion pack for Firaxis' superb Sid Meier's Alpha Centauri.

Strategy Plus summarizes all the goodies you get in their review:

"Alien Crossfire introduces seven new factions: The Data Angels (covert ops hackers); the Cybernetic Consciousness (cyborg technologists); the Cult of Planet (environmental extremists); the Free Drones (labor society); and the Nautilus Pirates (maritime dwellers.) Rounding out the list are two newly-arrived factions of an alien race known as the Progenitors, which engineered the lifeforms on Planet as part of a cosmic experiment in large-scale sentience. The Manifold Caretakers wish to preserve the original purpose of the Sixth Manifold?Planet?while the Manifold Usurpers see Planet as a resource to be exploited, in whatever way is convenient. These philosophies predictably put the alien factions at odds with each other, and with the human interlopers who find themselves caught in the middle.

In addition to the new factions, the game introduces a plethora of new tech advances, base improvements, and secret projects, complete with new voiceovers and movies. Many of these revolve around the aliens' "resonance" technology, like the new weapons and armor that make PSI combat a much fairer proposition in the early game. Several of the new base improvements enhance the exploitation of ocean resources, making sea bases much more attractive than before. Probe teams receive an added boost with the new Covert Ops Center, and with the Algorithmic Enhancement ability even the Hunter-Seeker Algorithm is no longer proof against probe attacks. Some of the new secret projects are destined for popularity? like the Cloudbase Academy, which counts as an Aerospace Complex in every base, or the Manifold Harmonics project, which confers rich production bonuses upon fungus and monolith squares.

That's not the end of the goodies Alien Crossfire has to offer: There are new special terrain features and new items in colony pods, including powerful alien Battle Ogre units. Fungal and tectonic payloads allow missiles to be used for "instant terraforming." There are even three new native lifeforms, including a submarine unit and an artillery piece that add some welcome variety to native unit combat. All of the new features are described in a comprehensive booklet that supplements the original Alpha Centauri manual.

The alien factions are fresh and fun to play. They have their own unit graphics and experience the game's fictional interludes from a completely different perspective. Instead of obtaining energy through trade, the Progenitors draw their income from an "energy grid" at a rate determined by the number of base improvements and secret projects they have built. The aliens do not participate in the planetary council and cannot achieve a diplomatic win; however, either alien faction can win by building a "Resonance Communicator" to summon reinforcements from their home planet. Because of their innate bonuses, the alien factions tend to dominate; to prevent them from getting too powerful, the game puts both factions in every game and permanently sets them at each other's throats. (Regrettably, an apparent bug allows games to be started with only one alien faction.) The game's AI seems to struggle even more with the human factions than it did pre-Crossfire, though they can still do well with a good starting position?this seems to be especially true of the Pirates, which are the unrivaled masters of the sea.

Alien Crossfire is a jam-packed expansion that seamlessly blends many new and interesting features into an already great game. The bugs are occasionally annoying, but not serious enough to prevent you from enjoying yourself. If you're a fan of the original Alpha Centauri, your return visit to Planet should be just as engrossing as your first." Highly recommended to fans of SMAC everywhere.

Note: This expansion pack requires SMAC to be installed. You can buy the original game directly from Firaxis here.