Mockup Designer is an online mockup creator.
License: Open Source
Apps available for Online
Fluid UI is a browser-based wireframing and prototyping tool developed by Fluid Software and used to design mobile touch interfaces.
License: Freemium
Fluid UI excels in real-time collaboration and ease of use, making it ideal for quick prototyping. In contrast, Mockup Designer offers more customization and offline capabilities, but has a steeper learning curve.