Mockplus is broader than a mockup generator: a full design and prototyping platform covering wireframing, prototyping, and design handoff, not merely static device mockups.
Since it spans wireframe through interactive prototype in one tool, it covers more of the process than a single-purpose mockup site, at the cost of being a heavier tool to learn.
Good for teams wanting one platform for prototyping and handoff rather than assembling separate tools for wireframes, mockups, and dev handoff.
It's a genuinely different category of product from most other entries here: evaluate it as a prototyping tool, not a quick mockup generator.
Mockplus's products (DT, Cloud, RP) each ship a free-forever plan: Cloud's Basic tier, for instance, covers 10 projects and 10 team members. Paid plans start around $5.95/month and raise those project, page, and seat limits.
All-in-one product design platform for prototyping, collaboration, and design systems.