If you're looking for a Mockitt alternative, Figma is a top option. It's an all-purpose design, prototyping and development tool with features like AI-generated UI, auto layout and real-time collaboration. Figma has a wide range of integrations with other tools and a mature design system, making it a good choice for a broad spectrum of users, from UX designers to developers.
Another option is Uizard, which offers AI-based UI design tools, including a text-to-UI generator and drag-and-drop editor. Uizard supports real-time collaboration and design sharing, so it's good for creating and iterating on product designs. It's good for creating mobile and web app designs, wireframes and prototypes.
If you're looking for a lightweight yet powerful tool, Visily could be a good option. It includes AI tools like Text to Diagram and Screenshot to Design, as well as more than 1,500 prebuilt templates and real-time collaboration. Visily is free to use, with a Pro version planned, so it could be good for teams trying to streamline their design workflow.