Dynamic Widgets allows you to select what widgets you want to display on which parts of your website. Easy as that.
It is amazingly complete and easy to use. You can show specific widgets for certain posts, pages, categories and A LOT more options, this gives you the ability to create very complex and interactive sites.
Works just perfect combined with other plugins like AdRotate, Types and Views.
As I said, this is one of my Top 10 plugins from all times. I use it in all my developments :)