Getting Open street map data

To generate x-plane data with osm2xp, the first thing you'll need is an open street map file.
Here are some place to get osm data :

Official Open street map website , world map where you can export small zones (5000 nodes max, osm files)
geofabrik, files for whole countries, regions...
cloudmade, another map repository, like geofabrik.

Osm2xp can read osm and pbf files.
Osm files are just xml files, the good thing with them is that you can open and edit them with a basic text editor, but they are a lot bigger and slower to read than Pbf files.