One of the things that prevents WordPress from becoming really powerful as a CMS is it's lack of ability to display context relevant related content next to the main content on a given page. You only have a handfull of widgets and typically they are all displayed on each and every page on your site.
Flexible Widgets is a plugin which makes it possible to display a widget on selected categories and/or pages only.
Just drop it in your plugins folder and it replaces your default WordPress widgets with new, flexible ones.
Oh, and by the way. This is my first plugin for WordPress, so I guess there is room for improvement. Please feel free to help me out on this project, if you like.