Easy Rectangles, Circles and Ellipses in QGIS
“Rectangles ovals digitizing” plugin by Pavol Kapusta adds editing tools that make it really easy to create rectangles, squares, circles and ellipses. These are the tools provided by the new plugin:
Tools in "Rectangles ovals digitizing"
Give it a try!
Free and Open Source GIS Ramblings 
Where can I download the “Rectangles ovals digitizing” plugin?
It’s available through the QGIS Contributed Repository at http://pyqgis.org/repo/contributed which should be enabled by default in Plugin Installer.