OpenTTD is a free and open-source "transport and urban planning" simulator game. OpenTTD is an open-source clone of the commercial Transport Tycoon Deluxe but with some extra features such as: multiplayer mode, custom AI selection, fully customizable map sizes, custom sound & music, new scenarios and graphics.
Mac OS X 10.4 or newer
Intel powered Mac
Internet connection for multiplayer
Free download at opened.org