Justin Tadlock has just released Query Posts Widget for WordPress 2.8+. This widget allows you to add a list of filtered posts to the sidebar (or any widgetized area) of your blog.
You can filter these posts by:
This may seem simple, but the ability to combine 30+ filters together provides you with unlimited possibilities.
WordPress Modder has created a great video that gives a rough overview of how the plugin works.
To go one step further you can combine this plugin with the Widget Logic plugin to show or hide it using WordPress Conditionals. For more information about this technique refer to WordPress Modder’s post Query Posts Widget Plugin + Widgetized Page Template + Widget Logic = WordPress Gold.