As a popular IDE for Java application, Intellij IDEA's look and feel is very beautiful in my opinion.
How did it do that? And how can I get a look and feel like Intellij IDEA in my Swing app?
Current IntelliJ IDEA versions come with 3 themes bundled:
Darcula theme is available as a stand alone open source project and can be reused in your own apps.
Original answer from 2011:
By default IDEA uses Alloy Look&Feel. It's a commercial L&F and it's already quite old and with no updates. One of the major issues with it is that it doesn't support anti-aliasing on most of the platforms.
Therefore, for Windows I'd recommend using Windows L&F. For Mac IDEA already uses Mac specific L&F. Alloy may be worth it only on Linux where most of the other L&Fs look very ugly, though GTK might be a better choice.