Back in the old days when I was a Java developer (sigh), I made extensive use of several Jakarta Commons packages. Lang and Math are pretty much self explanatory, but I found the BeanUtils and particularly Digester a bit harder to get into. Jason Menard has put together a couple of very nice introductory articles: Touring the Commons - part 1 (discussing BeanUtils) and part 2 (discussing Digester). I wish I'd had these when I was learning my way around these packages!

If you aren't using these packages, take a look. They are all really useful.

The other package that I had to fight my way into was Collections. Any nice tutorials out there for this?