When looking at the immediate area around a backlink, it is not unusual to see that the link is surrounded by other links. This would typically be seen in a header, footer, navigation, blogroll, or on a category hub page. Link Density is a measure of the percentage of surrounding content that is made up of other links. A high Link Density suggests that the link is indeed in the middle of a group of links, whereas a low Link Density implies that the link is surrounded by non-link content, for example, a single link inside a paragraph of text.
Here is the list