This is a repost of a post from an old blog, made on January 3, 2012, that used to be on:
The big disappointment to me was the fact that on single-post pages you don’t get to have sidebars, which made the page really odd looking to me, so I started looking how to add it through the editor directly in code of the pages.
I looked through several sites and blogs that tell you how to do that but then found that there’s no need to add any child themes or make any modification through the editor.
There is a simple WordPress plugin that lets you do that with only a few clicks:
Once you install it and activate it you will get Theme Extensions option under the Appearance section which lets add sidebar widget to single-post pages and pages, plus a few other options.
Hope that helps someone.