Does anyone know if there are any good methods or apps to access and download the metadata of OSM from desktop?
http://planet.openstreetmap.org has a changeset file you can download, which includes information about each change made to the database. Look for changesets-latest.osm.bz2.
If by "metadata" you mean vector data, then there are several ways to do it. But it depends on your needs.
These two ways allow you to specify a map area you're interested in (but the size of the area is limited):
- http://www.openstreetmap.org/export - choose "OpenStreetMap XML Data"
- Using Maperitive (see the first two steps).
If you need the whole world, download the planet.osm. You will however need a lot of HW resources, so good luck with that.
For inspiration, check out the blog posting "OpenStreetMap: a Year of Edits" by Derick Rethans (http://drck.me/osmyoe11-9nz). He discusses how he got hold of the changes in the OSM data set that happened in 2011 and how he made a beautiful video out of it. Intrigued to learn what use you are investiging!