tru: Jar files are linked to javaw by default and therefore can be executed by a doubleclick. they are however not linked to javaw if you use the JRE 1.3.1 (the one that is installed by limewire). i think that must be a bug, all java versions since 1.2 made jar files executable by doubleclick. if you like, you can manually link them to 'javaw.exe -jar "%1"'.

PS: indeed jar files are zip files. just another extension.