Sometimes I feel I just can't keep up with all the things that are going on with Java. Okay, I know I can't keep up with it. Like today, I found this Cool Stuff page at Sun (has a 'shaking' logo and all) (via this post to sjug-announce).
TLDDoc: Javadoc for your taglibs. (I once wrote an in-house Doclet that does a similar thing, but TLDDoc more well-rounded.)
jvmtat: performance analysis tools for 1.4.1+ JVMs.
This is a GUI client tool that provides access to the HotSwap functionality available in the Java HotSpot Virtual Machine starting from JDK 1.4. Using this tool, you can dramatically improve the time of the usual "test - find a bug - stop the program - change - recompile - restart the program" development cycle by removing the "stop" and "restart" elements from it. Furthermore, you can use it to patch "on-the-fly" deployed applications that need to run uninterrupted.