JS Paint is the remake of MS Paint that lets you edit images on the web.
Mockuper is a free online custom mockup generator web app to help you create mockups for your designs.
DesignCap is a free online poster maker that lets you create your own stunning posters with ease.
If you need a professional logo designed without any fees, DesignEvo lets you create logos online for free.
If you do not like to manually create icons of your app for multiple devices, then let App Icon Generator help you by letting you quickly create icons for all the supported devices in one go.
Inkscape is a free and open source graphics editor like Adobe Illustrator for Windows, Mac, and Linux.