We recently looked at the commenting practice of active working open source projects. It is quite impressive: The average comment density of open source is around 19%. (Comment density = comment lines / (comment lines + source code lines)). That is much more documentation than most people thought!
However, such a rough number needs discussion. Here, I look at the comment density on a programming language basis. As it turns out, the comment density of active open source projects varies by programming language. Java is leading the bunch, but that needs further discussion.