Open-source java profilers
Yesterday, I did some (successful) experiments with HAT, a simple yet efficient heap analysis tool for java. It doesn't work with JDK 1.4 though, apparently due to a bug in the JVM. Still very useful and simple to use.
I remember starting with java back in 1996, thinking that memory leaks were gone forever. They are mostly gone in java, but when someone programs one in it's sometimes even nastier than before, because people are not used to handle them anymore.