OS X extras
There are a few good resources out there to help people write Java apps that look better on OS X, e.g.
But it is all a bit disparate and little gems of wisdom - like setting rootPane.putClientProperty("apple.awt.brushMetalLook", Boolean.TRUE) - are really valuable to know about. (Note: although macwidgets looks great, their API design is totally non-standard making it next to impossible to use their widgets in any GUI Editor)
Is it possible to use SwingX to get benefits for free?