Simcity 4 Multiplayer
I've been playing a lot of Simcity lately.
I've had the idea for a while that I've wanted to do a multiplayer game with friends via Dropbox, so I roped a few people like Tate into it and we found it works surprisingly well.
After a few rounds of back-and-forth I took the liberty of rendering out a mega-screenshot and setting it up as a zoomify image. (The stitching was all done by hand, so there's a few rough edges around city borders.)
Simcity 4 is by far my favourite version of Simcity. It has a sweet 2.5D aesthetic (3D models are rendered down to 2d on a fixed trimetric orthographic projection), but more importantly it has a large community of modders and lets you build massive regions spanning multiple cities.
Newer versions in the Simcity franchise sacrifice detail for simplicity and tend to be more "town building" than "city building" sims. Of the newer games I prefer Cities XL (which I feel redeemed itself after a botched launch and a few re-releases), but it's difficult to go past the homely charm of Simcity 4.