Great use of annotations
C# entered the field of battle some seven years ago as a better version of Java. The one feature that many people referred to when claiming C# superior, was custom attributes. Now Java has them too, but they go under the name of annotations.
The ability to attach meta data to various parts of your program is a powerful way to integrate with frameworks or configure external utilities. Mike Kaufman, for example, used annotations beautifully to configure an Obfuscator. He has posted the details on his blog.