Bryan Shannon used to be part of Maxis, where he worked as an artist helping make buildings for the last SimCity. Now, in his spare time, he's crafting stuff for the game that followed in SimCity's wake.
One of the best things about Cities: Skylines, as I mentioned in my review, is the way that modding isn't just encouraged, it's baked right into the game's main menu, so people can add new buildings to the game with the click of a button. These buildings don't just look different, either; users can also modify their effects.
The first building he's completed is this replacement coal plant, which not only looks more like a coal plant than the game's weirdly sci-fi default design, but works more like one too (it generates more power than the default, though at the expense of more noise and pollution).
Here's why this plant (and the way mods are already being designed in general) is so cool: Shannon's description says "Overall, this coal power plant is a deviation from the original style. This asset introduces a nice dirty brown brick in to the game's overall neutral palette." Skylines looks great for the most part, but the city does indeed have a very neutral colour palette; mods like this will really help certain buildings stand out.
The second custom building he's added also adds some colour and personality to the game: it's a burger joint, complete with a backlot.
You can add Shannon's buildings to your own game here, while you can see more of his 3D art in general (including his work on SimCity) here.