Now that WordPress 2.2 comes standard with the sidebar widgets plugin (which allows you to drag and drop customize the sidebar areas without altering the theme files–look in the Admin Presentation area) as a core system, here are some other widget plugins that you can get that are pretty useful:
Configurable Pages Widget: This widget lets you set up the Pages sidebar tree menu in a better way than the default one does, you can exclude certain pages appearing and set the order http://is-here.com/projects/wordpress/pages
Drop Down Archive and Progress bar widgets: One widget adds a Drop Down archive to your sidebar area (much like my AWSOM Drop Down Archive plugin does on the main) while the other one adds a progress bar to your sidebar area that has color selection and other customizable features. http://tech.windsofstorm.net/
By now there are tons of widgets out there for almost anything. A quick Google search for WordPress widget will net you tons. So go check out what you can find, you never know what’s available.