Free!
Excellent PC graphics program, basically the version of Sketch for Windows. It also opens Sketch files!
Free with limitations
Graphic design tool designed for web and apps, a bit too limited in the free version.
Free with limitations
Excellent program for graphic design and prototyping with a focus on team collaboration and web experience. A reference point.
Free!
Interface design and prototyping software by Adobe. Lately It has become totally free. It started a few years later than his competitors but it is growing steadily. Great.
Free with limitations
Graphics software designed for wireframes, mockups, diagrams and prototypes. The free version is very limited as it cannot export the created works.