shazdeh

This plugin adds two options for widgets, one to set the width of the widget and also an option to show an icon beside the widget. You can define new rows of widgets using the Divider widget. Supports RTL layouts as well.

Widgets in Columns

Using this plugin you can show your widgets in desired columns and rows. You can also display an icon beside the widget.

Chad Hovell

Select extra layouts from the post editor Screen Options menu. 
There are now 24 variations to choose from!

Three Column Screen Layout

Three, four and five column screen layouts for the post editor.

Widgets in Columns vs Three Column Screen Layout