GIMP

GIMP is a free and open-source image editing software. It can be used as a simple paint program, an expert quality photo retouching program, an online batch processing system, a mass production image renderer, an image format converter, etc.
GIMP is totally a free image editing software worth trying for simple image editing tasks.

GIMP screenshot

Paint.NET

Paint.NET It features an intuitive and innovative user interface with support for layers, unlimited undo, special effects, and a wide variety of useful and powerful tools.

Paint.NET screenshot

ThunderBird

ThunderBird can be used if you like checking and replying to your web email from the desktop. It also allow you to check multiple Emails and apply filters to the incoming mail.

ThunderBird screenshot