I found this post by Carlos really interesting. Seeing the file size of the data for the whole world is kind of like seeing the size of all of wikipedia compressed, it fills me with amazement. Much of the things that have been built is in that data. It piqued my interest and got me looking into the OSM data model.
Also, NIMBY Rails is great for train nerds like me, y'all might like it too if you dont already.