I've used the Conditional Widgets plugin for some projects. I find it particularly useful when working on a site where contextually-relevant calls to action need to be easily updated by the site admin.

For example, say you're running a site for a bookstore. You have categories for different genres of books (fantasy, sci-fi, drama, horror, etc...). We could have a special promotions widget that only shows on posts in X categories.

Really rough example there, but you get the idea. :) Conditional widgets are a useful tool.