Justin Tadlock

The Query Posts widget was written to allow users that don’t know their way around PHP to easily show posts in any way they’d like. It’s like having a cool WordPress developer as a friend ready to do your bidding. Seriously.
The widget has over 40 options to choose from. You can list posts by category, tag, custom taxonomies, author, date, time, name, or anything you can imagine. You can choose to show the full content, excerpts, or even a simple list. You can order the posts in all sorts of ways. Oh, and you can even show pages.
This is the widget that keeps users out of the code and gives them the ability to display items on their site how they want.

Query Posts

A WordPress widget that gives you unlimited control over showing posts and pages.

dmallon

The Post To Sidebar plugin makes it easy to display post content in the sidebar areas of your site. Once the widget is activated, a multi-select dropdown of all your published pages appears on post editing screens. Select the pages upon which you want the post to be displayed and the post will appear on those pages.
There are options to hide the post title in the output and to show the content as an excerpt.

Post To Sidebar

A WordPress plugin/widget that gives you the ability to put content (posts and custom post types) in your sidebar.

Query Posts vs Post To Sidebar