I was going nuts trying to open a .jar file that I downloaded from the Azureus site. I kept on getting the same message from Jar Launcher: The jar file "[...].jar" couldn't be launched. Check the Console for possible error messages. I got a similar message when I tried to open several other .jar files I found on my disk. It was that problem that led me to this forum.
I tried re-installing Java, removing the Java Preferences, etc., and nothing worked. Then I noticed, examining the contents of the Azureus application package, that there were a number of .jar files in the various Contents/plugins/az* directories! I probably could have just copied the .jar file to one of those directories, but I went back to Azureus, selected the Plugins -> Installation Wizard ... menu item instead. The rest was trivial!
I wonder if there are any .jar files that really do run by clicking on them in the Finder, or by opening them with something called Jar Launcher that I can't even find on my computer (Tiger 10.4.6)! If so, it's rather misleading to use the same extension for files that can't be opened that way!