Javadoc
From APIDesign
Javadoc is one of the best inventions of Java. When we were working on the Xelfi student project (which later evolved to NetBeans), we needed not just a working IDE, but also some documentation. Java was the first language that allowed one to generate documentation from the code. So we did it - we generated hundred of pages of Javadoc. Most of the methods had no comments, but as the Javadoc tool wasn't widely known at that time, everybody was impressed and we passed the exams.