I genuinely don't understand why more people haven't rated this plugin 5 stars. It's simple, lightweight, and gloriously useful. Have you ever wanted a sidebar to display on some pages but not others? Maybe you want a certain banner ad in a text widget to display on the homepage, but not other pages? This is the setup I have on my site, and Widget Logic makes it as easy as a line of 'code' (I say code, but it's not like you need programming knowledge to use this plugin: snippets are provided in the FAQ) at the bottom of the widget.
Recommended. Don't know where I'd be without it, actually.