Artiss Content Reveal allows you to easily hide and reveal WordPress content, whether it's in the sidebar or in a post or page.
After searching for a method to hide content in my own site's sidebar I found that most of the solutions involved using widgets (which often provide less functionality compared to their direct function call equivalents). As well, they often make use of Scriptaculous, jQuery or mooTools to provide effects, but often add complexity and bloat.
For more information and advanced options please read the "Other Notes" tab.
Note for those upgrading from version 2 Version 2.1 includes new cookie code, including a new cookie name. To keep code size and risks of failure to a minimum no backwards compatibilty was added. The consequence of this is that, after upgrading, user's existing cookies will be lost.