Civ2: Multiplayer Gold Edition (MGE) or Ultimate Civ2: includes Fantasitic Worlds (FW) additions such as new event actions and triggers aswell as an Editor and has internet/LAN/Hotseat/PBEM options.
Civ2:Test of Time (ToT): includes 3 MODs, new events, multiple maps, some rules changes (mainly unit abilities) more units and techs and some changes to the graphics (the bitmap files as well as the new animated unit sprites).
Call to Power (CTP): similar, more MODdable but no events (that I know of), very glitchy. CTP2 is pretty much the same deal only fancier. Not particularly successful.
Civ3 (and all its expansions): is essentially just another version of Civ2 only with better graphics, slower gameplay, more features, no events and the units are animated.
Sid Miere's Alpha Centauri (SMAC): Civ2 on another planet. New unit abilities, some new features such customizing units, elevated terrain, etc.) units are animated. VERY popular for a while.
FreeCiv/C-evo: MODs of Civ2 program. Not official products.
That's about it.
