There have been many computer games that I fondly remember, from the days of the Commodore-64 with such classics as Ultima III and Reach For The Stars, to the present day monster computers with quad cores and gigabyte graphics cards with such titles as RIFTS, Diablo II, and Grand Theft Auto.
But through it all, I cannot think of another game that has changed how I game so much more so than Civilization IV. I played the hell out of the game for five years before realizing that I could mod it as well. A little more than a year ago, with a few friends taunting me to try and create my own content, I began to fool around with the maps, the models, and simple XML changes.
I now am the creator of three separate mods for Civilization IV: Multiverse, Extreme2, and the Age of Discovery. Each time I dip my fingers into the modding jar, I find some new delectable capability or nuance of play I never thought of before. I am creating so much new content that often my friends cannot keep track which game we are on or which mod where the new units will appear in.
I have a long way to go with this game, but the horizon is far beyond my reach. I am still enjoying and exploring all the possibilities of simple XML changes. Beyond that, there is the python coding and even further out is the C++ coding I could do with new sets of DLLs. I am happy at the present time of just creating new model variants, such as my recent additions of the Fiat 3000, an Italian light tank from World War I, or the English Hurricane the aircraft that truly won the skies over the UK during that Battle of Britain.