Section Widget: This plugin is basically an improved version of the built-in text widget. It allows you to display some arbitrary text or HTML on your site - but instead of showing it everywhere on your site, it allows you to limit its display to certain section(s) of your site, including fine-grained per-page control. This plugin features an easy-to-use widget interface and there is no coding involved (check out the screenshots). In terms of functionality, this is similar to Alan Trewartha's Widget Logic and Matt Gibbs's Widget Block plugin, but this plugin does not require any knowledge about WordPress' conditional tags. This is especially great for multiuser sites because giving out access to the theme editor or allowing arbitrary PHP code is often undesirable.

Widget Logic: This plugin gives every widget an extra control field called "Widget logic" that lets you control the pages that the widget will appear on. The text field lets you use WP's Conditional Tags, or any general PHP code. There is also an option to add a wordpress 'widget_content' filter -- this lets you tweak standard widgets to suit your theme without editing plugins and core code.

Widget Block: Create widgets that only display on specified pages.