
Transport Tycoon Deluxe / Open TTD (free open source remake)
Wow. What can I say? Few games have occupied my time as much as Transport Tycoon Deluxe has. Created by the legendary Chris Sawyer, who went on to create the RollerCoaster Tycoon series and Locomotion, Transport Tycoon Deluxe captures perfectly what makes the strategy-simulation genre magical.
In Transport Tycoon Deluxe you take the place of an entrepreneur in the transportation industry. You begin with few resources and can decide to get into planes, trains, busses, or boats, carrying things like people, raw materials, and goods across the land, sea, and air. (Hint: start with trains, it's the best way to get started IMO).
A few brave, dedicated TTD fans have recreated the game as an open source project known as Open TTD. It's free to play and someone has even created a browser version that you can play free any time you want on pretty much any platform (Mac, Linux, PC, etc.) without having to download or install anything.
This game is amazing and you should play it. Also, the music rocks.