This widget was necessary for one client a few years ago and Zack Design instantly noticed how useful it could be to many different types of customers. If you use the Multiple Category Selection widget at the bottom of the left sidebar of this website you’ll quickly see how powerful it is.
Useful for real estate agents, as a way to drill down using post categories as ‘tags’, and other applications, this widget is a must-have for users with lots of posts!
See the WP Smart Sort and S Category Permalink plugins listed in Complementary Plugins in the External Links in the left sidebar. These work quite well with my plugin.