Mediamodifier - Create beautiful designs and product mockups in seconds.
draw.io - Online diagramming application
Mockuuups Studio - Fast and easy way to create product mockups on macOS, Windows and Linux.
Dia - Dia is a GTK+ based diagram creation program for GNU/Linux, MacOS X, Unix, and Windows, and is released under the GPL license.
MockupBro - Create awesome mockups without any graphic software. Just choose mockup, upload your design and download your image.
yEd - yEd is a free desktop application to quickly create, import, edit, and automatically arrange diagrams. It runs on Windows, Mac OS X, and Unix/Linux.