I've been playing this on and off for a couple weeks now, actually, but Civilization 4: Caveman 2 Cosmos.
C2C is a mod for Civ 4 which... well. Huge would be an understatement. The current version (v22, and it gets updated about once a month or so) has 420 techs in the tech tree, over 500 different units, dozens more resources, has about doubled the list of available civs... The game now goes from 12,000 BC (learning how to smash rocks together and make grunting sounds mean something) well into the future, with techs like Time Travel (which is still several steps down from the end of the tech tree). They've also added "alternate history" techs like Megacavalry (Mammoth riders, Rhinocerous cavalry), Steampunk, and others. Hunters can subdue and domesticate animals (of which there are a huge variety), barbarian cities can develop into their own civs, great people can place resources on the map... There's 12 different categories of civics, with between 8 and 12 options for each... It's not even really a mod or expansion to Civ 4 any more, it's an entirely different game which just happens to play very similarly.
And it's extremely fun.