Note: The plugin is integrated into JOSM (expert mode). Hence, it is no longer available as a separate plugin.
The server that offer the OSM data, the OSM main API, has several independent read only mirrors. These mirrors have the same data, but can deliver it usually much quicker. The JOSM Plugin Mirrored Download allows you to use a mirror server. Uploads still go to the OSM main API, because that is the only way to keep the database in sync.
This manual refers to the version of 2012-02-02. Feel free to make suggestions for simplifications or extra functionality or report bugs to me <mailto:roland.olbricht(at)gmx.de>.
How to Install
Open JOSM and select from menu Edit > Preferences, then there Plugins (the plug symbol) > Download list. Check mirrored_download in the list. Now restart JOSM.
How to Use
Select File > Download from OSM mirror (Alt+Shift+Down). Then you can choose a download location with the well known dialogue window.
Note that the download of GPS traces isn't possible that way; you still need to download them from the main server.
If you want to use a specific mirror server, you can select one from the suggestions or you can enter a base URL yourself. The base URL must contain the part before map?bbox=... in that case.
The general server settings can be changed at http://josm.openstreetmap.de/wiki/MirroredDownloadInfo