Pencil

Open-source GUI prototyping tool.

Axure

Leading prototyping software.

MockFlow

Super-easy wireframing and UX suite.

Balsamiq

Rapid, effective and fun wireframing.

Justinmind

All-in-one prototyping tool.

Proto.io

Create interactive high-fidelity prototypes.

UXPin

Prototyping. Documentation. Design Systems.

Moqups

Create wireframes, mockups and prototypes.