No need for a plugin. Here's a simple way how to achieve the desired functionality:
Code:
<?php if (in_category('category-slug-1')) { ?>
your ad code for category 1
<?php } elseif (in_category('category-slug-2')) { ?>
your ad code for category 2
<?php } elseif (in_category('category-slug-3')) { ?>
your ad code for category 3
<?php } else { ?>
leave blank or insert general ad code for categories not determined above
<?php } ?>
Place it wherever needed. Put it in your theme's template or use a php ready widget.
Using category specific ads you can already target your advertising efforts quite well to increase conversions. I'm using methods like this one for all my autoblogs.
If I could help you, please hit the thanks button.