Open TTD puts you in charge of a transport company. You can create trains, planes, buses and boats as you aim to earn as much money as possible. You can choose to transport people or a variety of goods and each will require its own kind of vehicle.
Certain types of vehicles will need you to build roads and railways. Roads are normally simple but railways can get very complicated, with a variety of signal and station options. You can also setup a system of schedules and the game can be as simple or complicated as you like. Efficiency is the key to making money in OpenTTD, but it’s possible to make profitable routes without setting up mind-blowing track configurations.
OpenTTD is very obviously and openly based on Transport Tycoon, the classic Chris Sawyer game from 1994. That game is considered a classic of the sim/management genre and for good reason. OpenTTD takes the game’s core features and enhances them to make the game more playable and interesting.
You can choose to play on a random map or play through a scenario. The only scenario I’ve played so far is a giant, surprisingly accurate map of the UK. Building trains than ran for any significant distance meant a serious financial investment, and the scale made it very confusing, but if you wanted to put in the time this sort of large scale replication would be interesting.
If you enjoy sim or management games, this game is awesome fun. The map generation and variety of ways to approach the game mean that there are a huge number of ways to play. The multiplayer can be very fun, especially with people you know.
TL:DR: A well designed and wide open transport simulator. Easy to pick-up (when you use a tutorial) but with potential to be very complex. Play it here.