If you need OpenStreetMap data in bulk, you will need to download a dump of the database, known as a planet file. All planet files are hosted on http://planet.openstreetmap.org/, including the weekly full dump, the daily, hourly, and minute-by-minute change, or diff files.
There's no user interface to this site, only the raw directory listings. The latest weekly planet file is always at http://planet.openstreetmap.org/planet-latest.osm.bz2, and in each subdirectory containing diff files, there is a text file,
timestamp.txt, which contains the timestamp of the most recent diff.
If you don't need data for the entire planet, you can trim down the planet file to a particular geographical area, and some sites provide pre-built ...