That's the mac look-and-feel that is only (officially and legally) provided in the JDK (or JVM) downloads from Apple itself. ( However, since Java is platform independent, and the mac look-and-feel looks like any other to Swing, it is technically possible to download the JDK from Apple, and with some minor adjustments have the mac look-and-feel running with any Swing app. However, be aware that it is illegal to package Apple's plaf class files with your application if you are planning on doing this for anything other than personal use. ) There is also an Aqua theme pack you can use with SkinLF, if you just want the Mac OS X look-and-feel, but don't need the official one from Apple.
Write once, run anywhere, because there's nowhere to hide! - /. A.C.