Alpha Centauri is my favourite game, bar none, except Kingdom of Loathing, but KoL is a web browser game, single-person but with some multiplayer involvement.
I don't remember a Alpha Centauri demo anywhere, but I got my copy in a 10$ pack of games from The Real Canadian Superstore (Part of Loblaws, Maxi, etc.) along with Tiberium Sun, Celtics: rage of war, waterloo: napoleon's last battle, sid meier's civil war collection. Pretty good for 10$ Canadian... If anyone's interested, I'll consider buying + shipping some, but all real gamers already have one. =)