One thing that I have seen new-to-Java people miss is the SimpleDateFormat class. I found a bunch of legacy code on my current project that was written by a C++ programmer who didn't really know Java and he basically did all of the date-to-string formatting with custom code. Talk about re-inventing the wheel.
I recently started using the zip/unzip support that is part of the stock JDK. It works great! I'm using it to create KMZ archives in a webapp.
