Java Tools for Development and Testing:
appletviewer
The appletviewer utility comes with the Java SDK and serves as an alternate tool for testing Java applets if you don't want to be bothered starting up your browser of choice.
The appletviewer does not display any additional HTML code you may have in the HTML file containing the applet tag that accesses your applet, so it provides a more rudimentary test than your browser, but this will often be adequate, especially for initial applet testing, though you will eventually want to browser-test any "serious" applet, and probably with several different browsers, in fact.