Artboard Studio - Online graphic design app mainly focused on product mockups.

Vectr - Free vector graphics editor. A simple yet powerful web and desktop cross-platform tool for everyone.

Canva - Canva is a graphic-design platform with a drag-and-drop interface to create print or visual content while providing templates, images, and fonts. Canva makes graphic design more straightforward and accessible regardless of skill level.

Pixelixe Creative Automation - Auto-generate unlimited design variations of social media graphics and visual assets instantly.

Grid System Library - Free set of grids for iOS, Android and Bootstrap.

MockFlow - A super easy wireframing tool with all the other tools you need in the product design process