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.