Bloons Monkey City from Ninja Kiwi is a city building game with elements of tower defense, and unlike other titles in this genre offers plenty of customization. While it’s mainly a simulated game, there it a lot of strategic elements to it, and the graphics and gameplay are first rate.
OpenTTD is an open source business / transport simulation game based on the classic Transport Tycoon games. The difference is that this game is free courtesy of the OpenTTD Team. While the game is inspired by Transport Tycoon, OpenTTD offers more in terms of features. It’s your job to choose the routes so your cities get the services and goods they need, and that’s just for starters.