That's something I would be really interested to follow and maybe contribute to, so if anyone shares any pointers or links to existing OSS projects, I'd be very grateful.

Now necessarily city-simulation engine - just routing/pathfinding in graphs would be great.

The best example of a working and fun open-source routing/pathfinding engine is http://www.openttd.org

It's optimized for gameplay rather than perfect efficiency, but parts of it are quite hackable, including the ability to add custom AIs

I just started playing openttd, and am having a great time, though I still struggle with complex rail networks.

Do you have recommendations for AI? I've been using NoCAB, but I'm kind of frustrated with how that AI works, so I almost always play solo games now.

Thanks for sharing that, I had no idea that existed and I loved TTD!

Netlogo is an open tool written in Java for agent-based simulations (and much more), and quite mature, well worth checking out if you are into this sort thing. I always had a down on Logo because when I was a kid it seemed only to be mentioned in the context of turtle graphics, but the implementation here is powerful and reasonably terse.