Some popular tools for creating GUI mockups and prototypes include:

1. Sketch: A digital design tool for user interface (UI) design.

2. Figma: A cloud-based UI design tool for collaborative work.

3. Adobe XD: A user experience design software for creating and prototyping digital products.

4. InVision: A digital product design platform for creating and prototyping UI designs.

5. Axure: A wireframing and prototyping tool for creating interactive UI designs.

6. Balsamiq: A wireframing tool for creating low-fidelity UI sketches and prototypes.

7. (link unavailable) A web-based wireframing tool for creating low-fidelity UI designs.

8. (link unavailable) A web-based tool for creating digital product mockups and prototypes.

9. Flinto: A design tool for creating interactive UI prototypes.

10. Principal: A design tool for creating animated UI prototypes.

These tools offer various features, such as:

- Drag-and-drop interfaces

- Pre-built UI components

- Collaboration tools

- Interactivity and animation

- Feedback and testing tools

Choose the tool that best fits your design needs, skill level, and project requirements.