The Rails Port
The full README, along with the installation instructions and the contributing guide, are now contained within the code repository. See https://github.com/openstreetmap/openstreetmap-website
Before attempting to install The Rails Port be sure to review the complete list of OSM software components, in order to understand the role played by the Rails Port. For example, many of the features used by www.openstreetmap.org do not actually run on www.openstreetmap.org, but are still required for the full OSM experience. These other applications include the tile server (http://switch2osm.org/serving-tiles/, Ubuntu_tile_server) and a geocoder (Nominatim).
It is not recommended to set up local copies of The Rails Port for local data editing due to the complexity of later merging the dataset back into the main OSM database. For personal use, it is possible to use an offline editor such as JOSM or Merkaartor to do off-line editing to be saved to a .osm file. This can then be used with other OSM tools, such as Mapnik, and later imported to the main database if required.
See Rails port/UI for style-specific instructions after you've set up the code and want to start hacking on styles.