Category Icons is a way to improve navigation in WordPress

This is what John Pratt said in his post in WPHacks :

Having your categories a post is assigned to listed and linked is a great way to get visitors to view everything else you have posted in that category, but sometimes (like “ad blindness) readers are blinded to post meta info. Solve that by assigning images to your categories, so that they stand out prominently! All you need is the Category Icons plugin.

And I agree. :)