The first thing you might want to know about a link is what sort of content to expect from it. Linkclue icons help give you this information immediately.
Linkclue icons appear when you move the mouse cursor over a link, sitting to the right of the link text. Each icon provides a little extra information about the link, such as the related site favicon, what kind of file it is if it's not a normal Web page, and if it's going to open in a new window or tab.
Interclue supports a large number of Linkclue icons, and you'll have a good selection in the free download. Here are some common examples:
If Interclue thinks that a link is part of a local navigation bar or similar feature that has lots of links close together, it will display small icons in an attempt to avoid obscuring other links nearby. You can disable this setting if you find it inconvenient.
For more details, take a look at the Linkclues reference.