OpenTTDIt's less "casual" than most of the other games that have been listed here, but it's
freely downloadable to so it fits the thread description...


OpenTTD a free, open-source, improved reimplementation of the old
"Transport Tycoon Deluxe (TTD)" game, for Windows/Mac/Linux.
Unfortunately it has a pretty steep learning curve
(you will need to read tutorials on the wiki if you're completely new), but once you get to know it it becomes a perfectly good way to waste large amounts of time...

You have to build transport networks (railroad, buses/trucks, airplanes, ships) to transport passengers and cargo between towns and industries - and, of course, make a profit and outperform your (human or AI) competitors.
You can build simple train routes:

...or make it more complex to optimize efficiency:

...or go completely nuts:
(click for full size)