Daniel Ostman

Separator Widget inserts a horizontal rule ( <code><hr></code> ) or a custom div ( <code><div class="my-separator"></div></code> ) in your widget area. Each widget can be set to one of those options.
No extra styling is added so you are free to style them however you want it. 
Add your custom <code>hr</code> styling to your CSS or use <code>.my-separator</code> if you prefer that option.
Free and easy to use!
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Separator Widget

A widget that inserts a horizontal rule or a custom div.

mountbatt

This plugin adds a separator between the intro text and the extended (more…) text of the content of a post. It simply replaces the span id more tag with the separator that is set in the plugins php file.
Some separator samples are given there.
Upload into the /plugins directory and activate it in the Plugins Menu.
History:
1.1:
Fixed a bug that forced WordPress Adminmenu to show a white blank page.
1.0:
Initial Release

Add a Separator

Simple Plugin to add a separator between the intro and the (more...) content.

Separator Widget vs Add a Separator