This is neat, and almost completely useless. Cimtographer is a mod for Cities: Skylines that, among other functions, lets you export your city as OSM data that can be read by open source mapping applications like JOSM.
As an example, here's my little town of Jerkhole in all its growing glory.
The mod's other function is to allow players to import real-world road networks into the game. To be honest, I tried this and it all went horribly wrong.
This is how the mod interpreted the streets and terrain of London:
To be clear, this is probably my fault for doing something catastrophically idiotic. And, outside of real-world data, there are other potential uses. For instance, you could export one Cities: Skylines city, and then import its road networks seamlessly into a new one. Etc.
Advanced features aside, just being able to export and map your city is a nice touch. You can get Cimtographer from the Steam Workshop, and will find exported data in the Cities: Skylines root folder in Steam.