JDK 1.4.1 Beta
I thought Sun was slowing down in its releases of Java but just after releasing the JDK 1.4.0_1 update they've released 1.4.1 Beta. Not a blip on the Developer or normal Java web site.
The best feature is the Java logo while applets are loading.
Itanium support (Windows XP and RedHat Linux), Web Start integration (8MB JRE), bunch of javac bugs fixed (some of which I'd come across), ctrl-\ now shows deadlocks, and even more bugs fixed (notably NIO Linux bugs).