Paint.NET now supports a 'shapes' feature. Thanks to some very talented people at the forum, there is a 'shapes plugin' that can be added so you can make your very own shapes! That plugin can be found here.
I've made a large shapes pack of my own and preview images are posted below. My shapes pack can be found here. Simply download it and click the installer exe. You can choose to install a few or all of them. I hope you enjoy them!