OpenTTD is an open source simulation game based upon the popular Microprose game "Transport Tycoon Deluxe", written by Chris Sawyer. It attempts to mimic the original game as closely as possible while extending it with new features.
I've never been much of a player of the so-called "god-games". My ex-girlfriend used to be absolutely addicted to Sim City, but I never quite got the allure.
However, after recently playing the demo of Game Dev Tycoon, I became interested in the complexities of simulation-type games, even contemplating trying to write my own. In hunting for good sim-games for reference, I came across OpenTTD.
The first thing that attracted me to it, was the graphics - I'm a stickler for old-school games, and especially if they're old-school AND have good graphics, which this definitely does.
The gameplay itself is fairly intuitive and utterly, infuriatingly addictive. I started playing yesterday and didn't stop for 7 hours (even skipped dinner)...
Another thing that makes OpenTTD brilliant (and probably much better than the original), is the downloadable content. If the standard graphics don't appeal to you for some odd reason, I recommend you look into the "zBase Base Graphics" download (it's about 240MB though, so beware). There are maps, scenarios and endless downloads of all kinds.
This game is free, but I dare say that if it were being sold, it would be worth buying regardless of the price tag.