Freeciv

The Good
+ gameplay is as addictive as Civilization and Civilization II + very little gameplay stupidity (where players make the same oversight over and over, e.g. automatic dissolution of a city when it has no more workers in Civ II) + I like the interface much better (more gameplay oriented, fewer silly distractions) + playing with strangers from all over the world is great fun, and can give you great insights AI players just cannot teach you + the simple chatline interface is good enough for all kinds of off-game discussions + free, so you can download and be playing within a minute + some of the customizations (alternative settings or rulesets) create interesting new game variants + comes with source code, some of the minor features and bugfixes I managed to implement are now part of the game

The Bad
+ Getting started is far too difficult for newbies (although things are slowly improving) + Not enough human players, sometimes it takes too long to get a good game going + The AI isn't smart enough to be a challenge after about 100 games + Games take too long - long games are good, but short games should be possible too + Not enough interesting game variants have been developed considering the amount of customization possible (the "smallpox" problem)

The Bottom Line
A free, improved, multiplayer Civilization I and II. If you don't know those: a mix of chess and RISK , but better.