It's free and has many players online.
Runs on Linux, Windows and I think OSX as well.
Can you please, please, please, get Alpha Centauri working! We have alternatives to Microsoft Office. But Alpha Centauri, the best game in the Civ stable evah, is unclonable. I think I speak for most people here, as well as most people on this planet, and, indeed, most intelligent beings in our Universe, both biological, silicon, and sub-ethereal, when I say this is probably the single most important issue affecting humanity, and life itself.
Why not use the Linux version of Alpha Centauri ?
CCI Power 6/40: one board, a megabyte of cache, and an attitude...