Comment on WordPress Widgets Tutorial by SEO Dave.
After it’s enabled most widgets can be hidden or shown on specific posts, categories etc…
Go to the SEO Tutorial and scroll to just below the main content (below the author biobox) and you’ll see text links to all the SEO Tutorial articles, that’s a text widget with the links added manually and the display widget only showing the widget on the SEO Tutorial category. Go to any of the WPRobot articles or autoblog articles and on the left menu you’ll see an image ad to buy WPRobot (with a WPRobot Discount code), that’s a WordPress custom ad widget with the display widget only loading on articles related to autoblogging.
There’s also the Stallion Single Posts Widget, if you add that widget to a sidebar (under Appearance >> Widgets) whatever code you add on the edit Post/Page screen in the Stallion Single Posts Widget form at the bottom is loaded there. For posts you haven’t added anything on the edit screen, nothing is shown, ideal for one of bits of content you only want on one post.
This is good for when you want something specific on a lot of posts/pages, unique links or ads on a post by post basis. You can achieve the same with the Widget Display Plugin options, but if you wanted unique ads or something adding to 100 posts it would mean adding 100 widgets under Appearance >> Widgets which isn’t practical, with the Single Post widget you add the one widget and add the content on each posts edit screen.
I use the Stallion Single Posts Widget for quickly linking relevant posts together where I can’t be bothered looking for/adding relevant anchor text to add the links in the main content (contextual links are better, but more effort :-)).