Just came across this: the officially sanctioned set of icons designed specifically for use with the Java look and feel.
Seems rather a limited set but the documentation accompanying each icon is useful (giving the required keyboard shortcut key/mnemonic/tooltip text, etc.).
I have always found famfam coming to the rescue whenever I am iconically-challenged.
OpenClipart is another favourite; you just gotta love the little people:
[edit]
Found the Function Icon Set, another set of free, stylish icons.
