Edit 2: How about 30% off? Thanks k15866.
Edit: 20% off coupon stacks with this deal, goes from USD $11.99 to USD $9.59. Thanks tortyfoo
Runs on PC, Linux, Mac.
Some other games also on sale at GMG in "Massive Best of 2015" sale.
e.g. Dark Souls 2 USD $25.99 save 48%
http://www.greenmangaming.com/best-2015/
"Cities: Skylines is the best city-builder on the market right now. The game's presentation is stodgy, but it is all but guaranteed to provide you many hours of carefully crafting cities, laying out zoning, and establishing districts for specifics residential and industrial uses…all free from real-world mayoral headaches like 6 a.m. phone calls griping about snowplowing." [Gamespot ]
About the game:
Cities: Skylines is a modern take on the classic city simulation. The game introduces new game play elements to realize the thrill and hardships of creating maintaining the a real city whilst expanding on some well-established tropes of the city building experience.
From the makers of the Cities in Motion franchise, the game boasts a fully realized transport system. It also includes the ability to mod the game to suit your play style as a fine counter balance to the layered and challenging simulation. You’re only limited by your imagination, so take control and reach for the sky!
Paid about AUD$17 for this game about six months ago after watching Hex review it on Good Game. It's a great simulation, worth every cent.