Penpot

Pencil Project is an open-source GUI prototyping software designed to help create mockups across desktop platforms. The built-in shapes collection lets users draw different user interfaces from desktop to mobile platforms.

Mockups-Design is a collection site of many mockups in various categories. The design comes in PSD format that you can use commercially with no attribution.

Quant UX is a research, usability & UI prototyping tool to test your design and analyze interactive prototypes quickly. Its visual analysis makes it easy to find and quickly fix pain points in your UI design.

Mediamodifier is a mockup generator and graphic design editor that lets you quickly turn your ideas into inspirational marketing visuals using an intuitive drag-and-drop editor and many pre-design templates.

Mockup World is a mockups stock site offering paper mockups, picture frame mockups, packaging mockups, poster mockups, billboard mockups, iPhone mockups, and more.

Figma is a cloud-based UI prototype and design platform that lets multiple teams collaborate on designing, testing, and releasing interface or product designs.

NinjaMock is a wireframing platform that enables users to create professional mockups and wireframes quickly yet efficiently.