usage:
{{{
java -jar josm.jar <options>...
}}}
options:
{{{
--help|-?|-h                              Show this help
--geometry=widthxheight(+|-)x(+|-)y       Standard unix geometry argument
[--download=]minlat,minlon,maxlat,maxlon  Download the bounding box
[--download=]<url>                        Download the location at the url (with lat=x&lon=y&zoom=z)
[--download=]<filename>                   Open file (any file type that can be opened with File/Open)
--downloadgps=minlat,minlon,maxlat,maxlon Download the bounding box as raw gps
--downloadgps=<url>                       Download the location at the url (with lat=x&lon=y&zoom=z) as raw gps
--selection=<searchstring>                Select with the given search
--[no-]maximize                           Launch in maximized mode
--reset-preferences                       Reset the preferences to default
--language=<language>                     Set the language
}}}
options provided as Java system properties
{{{
-Djosm.home=/PATH/TO/JOSM/FOLDER/         Change the folder for all user settings
}}}
note: For some tasks, JOSM needs a lot of memory. It can be necessary to add the following Java option to specify the maximum size of allocated memory in megabytes:
{{{
-Xmx...m
}}}

examples
{{{
java -jar josm.jar track1.gpx track2.gpx london.osm
java -jar josm.jar http://www.openstreetmap.org/index.html?lat=43.2&lon=11.1&zoom=13
java -jar josm.jar london.osm --selection=http://www.ostertag.name/osm/OSM_errors_node-duplicate.xml
java -jar josm.jar 43.2,11.1,43.4,11.4
java -Djosm.home=/home/user/.josm_dev -jar josm.jar
java -Xmx400m -jar josm.jar
}}}

Parameters --download, --downloadgps and --selection are processed in this order.
Make sure you load some data if you use --selection.